Unique hues



In the opponent process theory of color, there are four unique hues – red, yellow, green, and blue – relative to which other hues are defined. Unique red is a red which appears to have no yellow or blue in it; unique yellow is a yellow which appears to have no red or green in it; etc.

Color names for these four colors are among the earliest defined in most languages.

Leonardo da Vinci defined these four as the basic colors from which other colors should be mixed.

Hues chosen as unique vary somewhat from person to person, but several models have attempted to define a standard. The Swedish Natural Color System is the most widely used of these, placing red, yellow, green, and blue in cardinal hue directions. The recent CIECAM02 model defines red as h = 20.14°, yellow as h = 90° , green as h = 164.25° , and blue as h = 237.53°.