Complementary colors are those which, when combined in some definite way, cancel each other completely according to some corresponding test. Complements in pigments and complements in colored lights are not the same. For example, complements in pigments are those which, when combined, completely neutralize each other and produce gray. Complements in light are those which, when combined, produce white light. Thus, when yellow and blue lights are combined they cancel each other and produce grayish white, according to the theory of psychologists, who work mainly with wave lengths and the effect of light upon the retina. But when yellow and blue pigments are combined they produce green, according to the theory of the artists who work with pigment and reflected light.
For convenience in use, the colors of the spectrum are generally arranged about a polygon or circle in such a manner that complementary colors appear opposite each other.