On the usefulness of basic colour coding in an information display

on the uedulNu of diq in an ingonmulien

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Harvey S Smallman* and Robert M Boynton

Basic colours segregate well in displays, but no better than non-basic ones equally well separated in colour space. Basic colour coding was compared with an individual's preferred code made up of a personal choice of colours. These codes yielded equally good segregation when assessed in a visual search task. However, when tested on another person's codes, with which they had had no previous experience, there was a suggestion that subjects were quicker to learn the basic than the idiosyncratic code. When coding qualitative data in a crowded display the authors advocate a code made up of the user's internally-generated set of basic colours. This code is easy to generate and obviates the need for complicated colour calibration procedures.

Keywords: colour coding, basic eolours, visual displays

Colour is well known to be the most effective way to label information on information displays or graphical user interfaces 1, but how does one choose a suitable set of colours to depict represented information which will maximize the benefits of perceptual segregation due to colour? Elaborate algorithms have been developed to enable display users to generate sets of colours maximally separated in various colour spaces 2. The authors take the somewhat heretical position that much of this effort is misdirected. In this paper an easy and effective way is revealed to select as m a n y as ten colours that are nearly optimal for coding purposes as evaluated in a visual search experiment. With this method, an excellent set of colours can be chosen without the need for physical measurement or laborious calibration procedures. Department of Psychology, University of California at San Diego, La Jolla, CA 92093-0109, USA *Correspondence address: Smith-Kettlewell Eye Research Institute, 2232 Webster Street, San Francisco, CA 94115, USA Paper received: 4 January 1993; revised: 14 June 1993

158

This method capitalizes on the categorical nature of h u m a n colour perception 3 and uses the concept of the eleven basic colours whose names were first discussed by Berlin and K a y 4. Each of these basic colour terms (white, grey, black, red, green, yellow, blue, orange, purple, pink and brown) refers to a distinct sensory experience. We would note that the existence and theoretical relevance of the eleven basic colours is controversial (for a recent critique of the concept of basic colours see Reference 5; for the opposite extreme that suggests that basic colour sensations may be associated with unique physiological substrates see Reference 6). Our interest in them is purely practical and driven by the fact that they work; the names of good examples of basic colours are elicited quickly, reliably and with good consensus by everyone with normal colour vision 7,8. In a previous publication 9 we reviewed the psychological literature, showing that colour coding is generally useful in visual search. We also provided experimental evidence that basic colours segregate well for this purpose. On a typical trial in our visual search experiment, there were as m a n y as 100 stimuli of up to ten different colours randomly scattered about the display, all of the same shape except for a so-called critical target. The critical target was one member of one of the groups of ten coloured items which were all of the same colour. If the colour of the critical target was known in advance, and was sufficiently different from those of the other colours in the display, search could be then be concentrated upon stimuli of that colour. Doing so greatly reduced the time needed to find the critical target because knowing the colour to be searched for causes stimuli of that colour to segregate, or 'pop out' perceptually from the clutter of other colours which can then largely be ignored. We also noted that it made no difference whether the colour to be searched was cued by presenting an actual specimen or merely the name of that colour.

0141-9382/93/030158-08 © 1993 Butterworth-Heinemann Ltd Displays Volume 14 Number 3 1993

Basic colour coding: H S Smallman and R M Boynton

In a control experiment, we established that it was not basic colours per se that expedited visual search. We concluded that basic colours segregate well because they are well separated from one another in three-dimensional subjective colour space. The evidence for this conclusion was the finding that another set of colours having approximately the same colour separation proved to be equally effective for expediting visual search. Although this result might suggest that the use of basic colours affords no special help in a visual search task, we speculated that basic colour coding might still confer certain advantages. First, would such codes be easier for viewers to set up than another set equivalently separated in colour space? Second, could viewers generate their own code yielding as good segregation as basic colours? Third, even if they could generate such a code would that personal code be too idiosyncratic for other display users to use it as effectively as another's basic colour code? The purpose of the present study was to answer these questions.

METHOD The study was divided into three distinct phases.

Phase 1: Setting the personal colours Subjects were shown several examples of typical search trials, in which the critical target was one of ten stimuli of the cued colour, but the only one with a distinctive geometric feature lacking in the other nine. These ten stimuli were presented among a large number of distractors of other colours. Subjects were able to experience how search is expedited when the colour to be searched is known. Subjects were told that they were to be asked to generate a set of ten colours and names for them, and that they would then be tested in the search experiment with these colours. Subjects were then presented with ten dark grey square patches arranged in a circle against the same grey background used in the search experiment on a CRT. Subjects were asked to adjust the colour of each patch in turn to be one of their preferred personal colours which would later be used in the search task. Subjects could then move through colour space by pressing certain key combinations on a keyboard and adjust the colour of the patch until they were satisfied with it. The patch whose colour could be altered was designated by a small black square next to it. Once that colour was set to the satisfaction of the subject the colour of the second patch was set, and so on until all ten were completed. On seeing all ten colours together, if subjects were not happy with any of their choices they were able to go back and change any colours as they pleased until they were completely satisfied. Subjects were continually reminded that the colours that they were setting were to differ

from each other to the greatest extent possible. Subjects were then asked to name each of their colours and these were recorded. That these names would be used to cue their colours in the search experiment was made explicit.

Phase 2: Setting the basic colours The procedure outlined above was then repeated but this time the subjects were asked to set the colours to those corresponding to basic colour names. This was accomplished by asking subjects to set a colour to, for example, 'the best blue you can imagine'. This procedure was repeated for each of the basic colours in turn (excepting grey, which was used solely to form the background for the search task). Subjects were then free to go back and alter colours in the same way as for their personal colours. Subjects were not asked to name these colours, which were already the optimal examples of a colour for each name. Subjects found this an extremely simple task, except for setting the colour brown, which of course has to set to a luminance darker than the background grey 1°. To aid those subjects who ran into difficulty, we suggested that they might first set a good orange (which they had no difficulty in doing) and then darken it. Those subjects who needed this help were generally startled by the results obtained by this procedure and all reported being highly satisfied by the brown that resulted. Subjects spent approximately the same amount of time setting their personal colours as their basic ones.

Phase 3: Testing the codes with the visual search experiment (experiment 1) The subjects were then tested with their two colour codes on a visual search task. The same search experiment and design were used to test subjects as reported before in experiment 1 with the stimuli used in experiment 4 of a previous publication 9, and therefore it is only briefly described here. Subjects were cued for 1 s with either the colour name or an example of the critical target for which they were to search. The critical target was a 1° square of the cued colour 'hollowed out' so that the centre was a smaller grey square, the same colour as the background (see Figure I ). This centre grey square contained a smaller black square adjacent to an identically sized white square. The critical target differed from the other distractor items of the same colour in that the orientation of tiny black and white squares was flipped. In the search display there were always ten examples of the cued colour, one of which was the critical target. There were in addition up to nine other categories of distractor targets of randomly determined different colours (which Displays

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Basic colour coding: H S Smallman and R M Boynton

Critical Target

Distractor Item

b

Colored

Background Grey

Figure 1 Schematic diagram to show the differencebetween a critical target item (left) and a distractor (right). These differed only in that the small

'domino' in the centre of the critical target was oriented with the white half on the left in the critical target and the right for a distraetor item. The 'domino' was set to a size small enough to prevent its discovery with peripheral vision, using the results from earlier psyehophysies9

were drawn from the colour code being tested) each with ten examples of that colour. So there were as few as ten or as m a n y as 100 targets to search through in the display. Subjects hit a key which erased the display to the background grey as soon as they had located the critical target and pressed another key to indicate its discovery or a third key to report finding no critical target. On 20% of the trials, determined at random, there was no critical target presented. This was necessary to keep subjects honest, for we did not require them to indicate the location of the critical target. On an additional 5% of trials subjects were cued with an 'X' instead of a colour. In this case they were obliged to search through all the coloured targets to locate the critical one (a so-called 'no information' trial). The display was erased after 20 s if no response had been made and this full time was often taken, particularly on these no information trials. Subjects were instructed that they could m a k e two types of errors, reporting locating a critical target when none was present (a false alarm) and reporting the absence of one when a critical target was indeed present (a miss). Subjects were instructed to respond as rapidly as possible while attempting to minimize the number of these errors they made. In addition to paying subjects a flat rate to participate in the study, a reward scheme was introduced for good search performance by the subjects. A prize o f $20 was offered to the subject with the fastest overall response time. There was another prize of $20 to the subject with the lowest overall number o f errors, and two further 160

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prizes of $10 to those subjects who came second in errors and response time as long as they did not perform excessively poorly in the other category. We have found in the past that such schemes tend to make for more attentive and productive subjects.

INSTRUMENTATION The stimuli were displayed on a Barco colour calibrator monitor, driven by a Radius GS/C 8-bit video card and a Macintosh II computer. The background was a video simulation of the '20% grey' used in our earlier colournaming experiments 7 made by matching an actual reflecting sample and the video representation of it. The luminance o f the background grey was ~ 28 cd/m 2. The 1931 CIE coordinates of the grey were x =0.354, y = 0.367. At the subject's viewing distance of 57 cm, 1 cm on the screen corresponded to 1° of visual angle. As noted above, stimuli were 1° x 1° square regions mostly filled with the appropriate colour. Stimuli were spotted randomly in the 2 1 ° x 28 ° central region of the screen, leaving a grey surround of 1.5-3 °. No other surround was used, and the r o o m was dimly illuminated. A small square region (33 x 3 3 m in(arc)) was removed from the centre of each of the filled squares, and replaced by the background grey, see Figure 1. Two small (2 x 3 pixel) elements near the centre of this grey region were altered. For most stimuli, the left element was black and the fight one was white. For the

Basic colour coding:

H S Smallman and R M Boynton

Table 1 Results of combined trials in experiment 1

RESULTS

Subject

Subjects varied considerably in their overall skill on the search task. The results for all trials combined are shown in Table 1. It is clear from inspection that there is no negative correlation between response time and errors (a speedaccuracy trade-off); in fact, subject GM exhibited the longest mean response time despite making many more errors than anyone else. He was also very tense during the experiment, perhaps because he seemed very much concerned about earning one of the rewards (which he failed to do). His data were very noisy and erratic. We have chosen to exclude his results from the remainder of the analysis, and he was not asked to participate in a second experiment to be described later. Note that the error rates for all subjects were far below the 20% error rate that would have resulted if a subject had guessed ‘target present’ after each trial. No significant differences were found when performance was broken down according to whether basic or personal colours were used (Table 2). Figure 2 depicts, for each subject, mean response time as a function of the number of different colours in the display. Functions are plotted separately for the four combinations of basic and personal colours versus the use of names versus exemplars for cueing the colour of the critical target. If a subject were able to completely ignore all stimuli of irrelevant colours for one of these conditions, the data would scatter around horizontal straight lines. In fact, there was a significant increase in reaction time with the number of colours in the display across conditions when the results were analysed with a two-way split-plot ANOVA (F(9,36) = 13.663, p < 0.0001). However, when this significant increase is depicted as only a 36% growth in reaction time with a 1000% increase in the number of distracters present then it can be seen that the basic and personal colours segregated remarkably well. If the four conditions of the experiment produced significant differences in response times, the four functions should be separated vertically and/or have different slopes. It is clear from inspection

MB BL CM SC GM

RT (s) 1.84 2.09 2.13 2.46 2.58

E (%) 1.7 4.3 1.1 3.2 6.9

[RT, mean response time; E, percentage of errors over the ten experimental runs.]

critical target, these white-black relations were reversed. This target configuration was chosen on the basis of ‘previous psychophysical observations’ which revealed that with this arrangement, something close to direct fixation was required to discriminate the critical target from other stimuli of the same colour. The fact that subjects were often still searching for the critical target after 20 s on many no-information trials gives support to this contention. The visibility and discriminability of the critical black and white elements was largely indepenent of the main colour of the stimulus which surProunded the central grey area. This is an attractive feature of the stimulus design, for in other colour search tasks in the literature it is not always the case that the critical target visibility is independent of the colour defining that target.

SUBJECTS AND DESIGN Five subjects aged between 20 and 30 participated in the experiment, one male and four females. Four were undergraduate students approximately 21 years old; the Bfth was a 27-year-old staff member in the psychology department. All were screened for normal colour vision using the Farnsworth-Munsell lOO-hue test. All subjects urere naive concerning the purpose of the experiment beyond what was revealed by the instructions. At the termination of the experiment, they were given their results and any prizes due to them, and the overall purpose of the study was explained to them if they requested such information. Each subject completed ten experimental sessions of 200 cued trials in the order B-P-P-B-B-P-P-B-B-P, where B represents cueing with basic colour examples or words (randomly distributed trials throughout the session), and P represents cueing for personal colour examples or words. Given that the probability of a no-critical-target trial was 0.2, and that for a no-cue trial was 0.05, there were on average 250 trials per session (with the expected variability around this mean). Except to keep the subjects honest, these extra trials are not of Interest and response times on such trials are not presented here.

Table 2 Performance using basic and personal colours Subject

Condition

CM

Basic Personal

RT (s) 2.13 1.93

MB

Basic Personal

SC

Standard error (s)

E (%)

0.072 0.063

1.0 1.2

1.88 1.80

0.042 0.047

2.0 1.4

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2.37 2.51

0.056 0.081

3.0 3.4

BL

Basic Personal

2.03 2.15

0.041 0.051

5.6 3.0

Mean data

Basic Personal

2.13 2.11

0.040 0.044

2.9 2.3

[RT, mean response time; E, percentage of errors.] Displays

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that there was no significant difference between the personal and basic colours, and the A N O V A confirmed this ( F ( 1 , 4 ) = 0.119, not significant). Search trials cued with examples o f the critical target were successfully 162

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completed faster than those o n which names o f the colours were provided ( F ( 1 , 5 ) = 15.335, p < 0.05) but the size o f this effect was small (only 100 ms) c o m p a r e d with m e a n reaction times o f just over 2 s.

Basic c o l o u r coding: H S Smal l man a n d R M B o y n t o n

Table 3 Suppliers of colour names in experiment 2 Subject tested BL CM MB SC

Subject who supplied colour names and exemplars SC MB GM CM

All subjects showed some improvement in performance as the experiment progressed, this being greatest for CM who exhibited a very large reduction in response time of about 3 s from the first to last block of 50 trials. Although CM had the longest response times for the first block of 50 trials, she approximately tied with two of the Other three subjects with the shortest response times during the fifth and last block of 50 trials. The other three subjects improved at a much slower rate, cutting off only about 1 s from the first to last block. Subject SC, because she started with longer response times and also improved slowly, finished with the longest overall response times of the four subjects, but in no case were 'there any significant differences in any of these learning functions. The names given to describe the personal colours were as follows: CM: light fuchsia, lime green, bright orange, medium blue, purple, dingy yellow, aqua blue, red, forest green, sapphire blue. MB: Lime, coral, purple, powder blue, lavender, green, rusty red, white, grey, black. SC: Fluorescent orange, mint green, ocean blue, lavender, hot pink, purple pink, baby blue, olive, red, chartreuse. BL: Fluorescent orange, neon green, electric blue, teal, purple, chartreuse, lime, periwinkle, black, faded peach.

continual goading of the experimenters for them to choose well separated colours. It is also likely, given that subjects were free to choose names in the first phase of the experiment, that they picked names that made sense to them, and for this reason they were able to perform as well when cued with names as with exemplars. On the other hand, whether their names would make as much sense to anyone else is another matter (see below). It seems likely to us that the setting of basic colours in the second phase of the experiment was not significantly influenced by the activity of the first phase, although we cannot prove this given the design of the experiment. If so, the use of basic colours would still have the advantage that they can be quickly chosen, avoiding the laborious process of the first phase. An additional possible advantage of using basic colours remains if it can be shown that there is a greater agreement among subjects for the names of basic colours (and the exemplars they represent) than for the idiosyncratic exemplars, and their names, provided in phase 1. Experiment 2, which had not been anticipated at the outset, was designed to test this hypothesis. We were able to recall the four subjects for further testing approximately 12 weeks after the initial experiment.

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DISCUSSION The results just presented show that it made no difference whether the search experiment was conducted using the colours set by subjects and then named, or with the basic colours set according to basic colour names supplied by the experimenter. If it were true that the separation of the colours in three-dimensional colour space was the critical variable for performance in the search experiment, then it must be concluded that the subjects did a very good job when they set their colours in the first phase of the experiment. Perhaps they were able to do this because they had the search experiment clearly in mind when they set their colours, because of preliminary experience with the search paradigm and the

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Basic colour coding: H S Smallman and R M Boynton

EXPERIMENT

2

This search experiment was conducted exactly as that of experiment 1, except that the personal names, and exemplars of both kinds, were those supplied by one of the other subjects determined at random (Table 3). Two sessions were combined, yielding 400 cued-colour trials, 100 for each of the four combinations of personal versus basic colours and names versus exemplars as cues. The results of experiment 2 are shown in Figure 3 with the data of the four subjects combined. Mean response time is plotted for the first, second, and final thirds (33, 33 and 34 trials for each condition) of the experimental session. It appears that subjects accelerated their responses over the course of the two sessions, although this effect did not reach significance in a three-way split-plot ANOVA (F(2,6)= 2.169, p < 0.2, not significant). By the beginning of the last third of the session, approximately 15 minutes experience, there had been sufficient improvement in using the cues for the initially difficult conditions that no difference between conditions remained. There is also a suggestion that subjects were initially slowest on trials on which the personal colours of another were provided them and quickest with the basic colours of another, although again, unfortunately, the three-way interaction did not reach significance (F(2,6) = 0.358).

C O N C L U S I O N S AND F I N A L D I S C U S S I O N When we began our research on colour coding in information displays, we hypothesized that there might be something very special about the use of basic colours that would make them segregate exceptionally well, thus making them outstandingly effective in a cued search experiment. This is clearly not so: basic colours do work well, but once again we have demonstrated that other sets of colours can be chosen that segregate just as well. The critical variable seems to be the sensory difference between the colours, regardless of the named categories in which they fall. In our previous work, we carefully selected a set of basic colours for video presentation using a method in which the video colours were matched against reflecting samples that were known optimal examples of basic colours based on extensive previous experimentation with reflecting samples7. One objective of the present study was to determine whether exemplars of basic colours created by individual subjects, achieved by comparison only with internal subjective standards, would also segregate well in a search experiment. They do, but once again we have found that subjects can select a group of personal colours (including some basic and some non-basic) which work just as well, at least for them. However, when presented with a non-basic colour name in the search experiment where the name is one 164

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that has been determined by a different subject, there is a loss of performance as revealed by initially increased response time, although this conclusion must be tem, pered by the fact that this effect failed to reach statistical significance. With a short period of training in another's personal colours and the arbitrary names given to them, these will eventually provide effective cueing. Our results suggest that, if one desires to employ ten colours that will segregate well in a video display, an ~ y and effective procedure for obtaining them would be to have any motivated individual with normal colour vision set ten basic colours against a grey background, just as these subjects did. (It may be better to average the settings, say one gun's value at a time, from three or four individuals because doing so would be expected to provide exemplars of basic colours closer to population consensus. As long as the resulting colours still appear to be good exemplars of the basic colours then we anticipate that the code will function well.) Basic colours confer an obvious advantage to users with international needs. With global economic markets now a reality, language remains the sole barrier to complete commmai, cation; basic colour coding could help reduce linguistic problems, for 'red' basically coded information on a display will be prototypical for American and Japanese users alike, for example. Basic colours also have the advantage that their names are meaningful without training. Our results should have implications for reflective displays as well. Good examples of basic colours could easily be selected from a colour atlas. For example, suppose that information falling into ten categories was kept in ten shelved notebooks. If the notebooks were produced with covers that provide good examples of basic colours, then the instruction, say, to 'bring me the pink notebook' would have unambiguous meaning, and the notebook could be found in an instant even if the ten notebooks were randomly arranged. Furthermore, once the meaning of the pink notebook was learned (and our data suggest that such learning is clearly possible), the instruction 'bring me the Smithson file' could immediately connote the pink notebook, which could then be quickly found without reading its label. We would make clear that basic colour coding is no panacea. We have been concerned in this paper with the use of coding maximally to delineate coded items for the purpose of locating them quickly. This is appropriate for coding qualitative items on displays such as folders on a computer desktop or different members of a category of items. However, if one is coding quantitative items such as temperature or ocean depth with colour then basic coding would probably not be desirable. A chromatic gradation within hue or colour category may be more appropriate for representing such information; Tufte gives some beautiful examples of such codes H. In Tufte's own words, we are concerned with 'colour as noun 'll, where colour labels specific objects and where those objects are categorically distinct from others

Basic colour coding: H S Smallman and R M Boynton

represented. Hence, basic coding is excellent for a specific airline on a screen where it can be linguistically and perceptually uniquely identified but p o o r for depicting a region of 95°F or 1000 feet sea depth on a map. For emissive displays, it is important to note that the full set of basic colours cannot be produced unless the display contains an intermediate grey background, because lightness relative to such a background, as well as chromaticity, varies for the basic colours. In addition, basic colour coding m a y appear somewhat garish to some (as others have noted, see Reference 2) but we are solely concerned with practical benefits of rapid search times here. We have also shown that, if subjects are trained to understand the need for good colour segregation in a search experiment, they are capable o f setting ten colours without recourse to the concept of basic colour names, and that they can then name them and use the names effectively when cued by them in a search experiment. However, until individuals have had experience with such idiosyncratic names, they are better at using their own personal names than those o f someone else. I f more than ten colours are desired, we demonstrated previously 9 that colours can be set that are intermediate between the basic ones (e.g. a balanced blue-green) and that these are effective, although it must be kept in mind that doubling the number of colours necessarily reduces the sensory distance between them, so some increase in response times will occur. The results of the present experiment suggest that exemplars of such a larger population of colours could be effective cues immediately, and that names could be readily learned to

represent the intermediates if cueing by names was desired.

ACKNOWLEDGEMENTS This research was supported by a McDonnell-Pew graduate fellowship in Cognitive Neuroscience to H.S.S. and by N I H grant E¥-01711. The authors would like to thank the two anonymous reviewers whose comments strengthened the paper.

REFERENCES 1 Christ, RE 'Review and analysis of color coding research for visual displays', Human Factors 1975, 17, 542-570 2 Travis,D Effective Color Displays: Theory and Practice Academic Press, London, 1991, Chs 3 and 4 3 Boynton,RM, Fargo, L, Olson, CX and Smallman, HS 'Category effects in color memory'. Colour Res. Appl. 1989, 14, 229-234 4 Berlin, B and Kay, P Basic Colour Terms, University of California Press, Berkeley, CA, 1969 5 Rather, C 'A sociohistorical critique of naturalistic theories of color perception'. J. Mind Behav. 1989, 10, 361-372 6 Ratliff, F 'On the psychophysiologieal basis of universal color terms'. Proc. Am. Phil. Soc. 1976, 120, 311-330 7 Boynton,RM and Olson, CX 'Locating basic colours in the OSA space'. Colour Res. Appl. 1987, 12, 94-105 8 Boynton, RM and Olson, CX 'Salience of chromatic basic color terms confirmed by three measures'. Vision Res. 1990, 30, 1311-1317 9 Smallman, HS and Boynton, RM 'Segregation of basic colours in an information display'. J. Opt. Soc. Am. (A) 1990, 7(10), 1985-1994 10 Bartleson, CJ 'Brown'. Colour Res. Appl. 1976, 1, 181-191 l 1 Tufte, ER Envisioning Information, Graphic Press, Cheshire, CT, 1990

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