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Complementary color

colours are called complementary if, when mixed, they produce a shade of grey (any shade between black and white).

The most important colours are the additive and subtractive primary colours, because these can be mixed to form all other colours. The additive primary colours are those that can be added together to make all other colours, as is done when light is emitted from the screen of a television set or computer monitor. The subtractive primary colours are those that, when mixed, subtract light from white to make all the other colours. This is what happens when pigments are mixed in printing and painting. The primary colours are listed below in three pairs, where each additive primary colour is shown next to its complement in the list of subtractive primary colours.

additive primary colour	subtractive primary colour
red	cyan
green	magenta
blue	yellow

Complementary colours in painting

In colour painting, there is a long tradition of using red, blue and yellow as the three primary colours. It is now known that these are not true primary colours, because they cannot be mixed to produce the entire range of colours visible to the eye. However, some artists continue to use this old version of colour theory. In this system, the complements of the three primary colours (called secondary colours) are as follows:

primary	secondary
red	green
blue	orange
yellow	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 colours in RGB color spaces

In an RGB color space (one in which the primary colours 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:

<math> (R,G,B) + (1-R,1-G,1-B) = (R+1-R,G+1-G,B+1-B) = (1,1,1). <math>

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