Two colors are called complementary if, when mixed in an additive system, they produce a shade of gray. In a subtractive system, two complementary colors mix to produce the color of the illuminant. Examples of complementary pairs are:

• red and cyan
• green and magenta
• blue and yellow

Complementary colors in painting

Because of the limited range of colors that was available throughout most of the history of art, many artists still use a traditional set of complementary pairs, including:

• red and green
• blue and orange
• yellow and purple

The best way to remember which colors are complementary is to look at a color wheel -- the complementary colors will be opposite one another. The use of complementary colors is an important aspect of aesthetically pleasing art and graphic design.

Complementary colors in RGB color spaces

In an RGB color space (one in which the primary colors are red, green and blue), a color can be represented as an ordered triplet (R,G,B) of color coordinates. This system is used in almost all computer displays and in many television and video systems. Let (0,0,0) represent black, and (1,1,1) represent white. Then, given a color (R,G,B), its complement is (1 − R, 1 − G, 1 − B). This way, when the two complementary colors are added, the result is pure white:

(R,G,B) + (1 - R,1 - G,1 - B) = (R + 1 - R,G + 1 - G,B + 1 - B) = (1,1,1).

The RGB color space is additive, which means that the color made by superimposing two colors is the sum of the RGB triplets of the individual colors. In such a color space, the sum of the three primary colors is white.