In theory, complementary colours, when mixed, will complete the visible spectrum: thus, two light rays of complementary colour will produce white, and two paints or inks of complementary colour will produce black or grey.
In practice, complementary colours are defined as colours that have maximum contrast for each other. The fundamental complementary pairs for painters are red/green, yellow/violet, and blue/orange, but each intermediate colour also has its unique complementary. The colour wheel arranges all the colours of the visible spectrum so that complementary pairs are opposite each other.
In painting, complementary colours are used for their vibrant contrasts and mutual enhancement when juxtaposed, for 'shot' or cangiante draperies, and for shadows tinged with the complementary of an adjacent highlight - a device imitating the physiological response of the human eye and much used by the Impressionists and Post-Impressionists.