Editing Color code (section)

===Categorical===
When color is the only varied attribute, the color code is ''unidimensional''. When other attributes are varied (e.g. shape, size), the code is ''multidimensional'', where the dimensions can be ''independent'' (each encoding separate variables) or ''redundant'' (encoding the same variable). Partial redundancy sees one variable as a subset of another.<ref name="CHRIST75"/> For example, [[playing card suit]]s are multidimensional with color (black, red) and shape (club, diamond, heart, spade), which are partially redundant since clubs and spades are always black and diamonds and hearts are always red. 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 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|pmid=13964770 }}</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 [[eight]] categories with optimized stimulus spacing along the hue dimension,<ref name="JONES62"/> though this would not be color blind accessible. The [[IALA]] recommends categorical color codes in seven colors: red, orange, yellow, green, blue, white and black.<ref>{{cite book |last1=IALA |title=The Surface Colours used as Visual Signals on Aids to Navigation |date=December 2009 |publisher=International Association of Marine Aids to Navigation and Lighthouse Authorities |pages=7–9 |edition=2}}</ref> 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. Adding color as a redundant attribute to a numeral or letter encoding in search tasks decreased time by 50–75%,{{r|CHRIST75|at=Fig9}} but in unidimensional identification tasks, using alphanumeric or line inclination codes caused less errors than color codes.{{r|1=JONES62|2=CHRIST75|p2=19}}

Several studies demonstrate a subjective preference for color codes over achromatic codes (e.g. shapes), even in studies where color coding did not increase performance over achromatic coding.{{r|CHRIST75|p=18}} Subjects reported the tasks as less monotonous and less inducing of eye strain and fatigue.{{r|CHRIST75|p=18}}

The ability to discriminate color differences decreases rapidly as the [[visual angle]] subtends less than 12' (0.2° or ~2&nbsp;mm at a viewing distance of 50&nbsp;cm),<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=129–135 |doi=10.1364/JOSA.48.000129|pmid=13514579 |bibcode=1958JOSA...48..129B }}</ref> so color stimulus of at least 3&nbsp;mm 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"/>