Color code

A color code is a system for encoding and representing non-color information with colors to facilitate communication. This information tends to be categorical (representing unordered/qualitative categories) though may also be sequential (representing an ordered/quantitative variable).

The earliest examples of color codes in use are for long-distance communication by use of flags, as in semaphore communication. The United Kingdom adopted a color code scheme for such communication wherein red signified danger and white signified safety, with other colors having similar assignments of meaning.

As chemistry and other technologies advanced, it became expedient to use coloration as a signal for telling apart things that would otherwise be confusingly similar, such as wiring in electrical and electronic devices, and pharmaceutical pills.

A colour code represents a variable that may have different forms. The colour code type should match the variable type:

The types of color code are:

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. For example, playing card suits 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 (connotatively or denotatively), versus search tasks, where a color stimulus must be found within a field of heterogenous stimuli. Performance in these tasks is measured by speed and/or accuracy.

The ideal color scheme for a categorical color code depends on whether speed or accuracy is more important. 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, 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. Adding redundant coding of luminosity and colorfulness adds information and increases speed and accuracy of color decoding tasks. 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%, but in unidimensional identification tasks, using alphanumeric or line inclination codes caused less errors than color codes.

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. Subjects reported the tasks as less monotonous and less inducing of eye strain and fatigue.