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The ideal color scheme for a categorical color code depends on whether speed or accuracy is more important.<ref name="JONES62">{{cite journal |last1=Jones |first1=Mari Riess |title=Color Coding |journal=Human Factors: The Journal of the Human Factors and Ergonomics Society |date=December 1962 |volume=4 |issue=6 |pages=355–365 |doi=10.1177/001872086200400604}}</ref> Despite humans being able to distinguish 150 distinct colors along the hue dimension during comparative task, evidence supports that color schemes where colors differ only by hue (equal [[luminosity]] and [[colorfulness]]) should have a maximum of 8 categories with optimized stimulus spacing along the hue dimension,<ref name="JONES62"/> though this would not be color blind accessible. Adding redundant coding of luminosity and colorfulness adds information and increases speed and accuracy of color decoding tasks.<ref name="JONES62"/> Color codes are superior to others (encoding to letters, shape, size, etc.) in certain types of tasks. However, in tasks that require rapid and precise identification, numerals or line inclination may be superior to color.<ref name="JONES62"/>
Tasks using categorical color codes can be classified as identification tasks, where a single stimulus is shown and must be identified ([[color task|connotatively or denotatively]]), versus search tasks, where a color stimulus must be found within a field of heterogenous stimuli.<ref name="JONES62"/><ref name="CHRIST75"/> Performance in these tasks is measured by speed and/or accuracy.<ref name="CHRIST75">{{cite journal |last1=Christ |first1=Richard E. |title=Review and Analysis of Color Coding Research for Visual Displays |journal=Human Factors: The Journal of the Human Factors and Ergonomics Society |date=December 1975 |volume=17 |issue=6 |pages=542–570 |doi=10.1177/001872087501700602}}</ref>
The ability to discriminate color differences decreases rapidly as the [[visual angle]] subtends less than 12' (0.2° or ~2mm at a viewing distance of 50cm),<ref>{{cite journal |last1=Bedford |first1=R. E. |last2=Wyszecki |first2=G. W. |title=Wavelength Discrimination for Point Sources |journal=Journal of the Optical Society of America |date=1 February 1958 |volume=48 |issue=2 |pages=129 |doi=10.1364/JOSA.48.000129}}</ref> so color stimulus of at least 3mm in diameter or thickness is recommended when the color is on paper or on a screen.<ref>{{cite book |last1=Conover |first1=Donald W. |last2=Kraft |first2=Conrad L. |title=The Use of Color in Coding Displays |date=1958 |publisher=Wright Air Development Center, Air Research and Development Command, United States Air Force |language=en}}</ref> Under normal conditions, colored backgrounds do not affect the interpretation of color codes, but chromatic (and/or low) illumination of surface color code can degrade performance.<ref name="JONES62"/>
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