Complementary colors are pairs of colors which, when combined or mixed, cancel each other out (lose hue) by producing a grayscale color like white or black. When placed next to each other, they create the strongest contrast for those two colors. Complementary colors may also be called "opposite colors".
Which pairs of colors are considered complementary depends on the color theory one uses:
Modern color theory uses either the RGB additive color model or the CMY subtractive color model, and in these, the complementary pairs are red–cyan, green–magenta, and blue–yellow.
In the traditional RYB color model, the complementary color pairs are red–green, yellow–purple, and blue–orange.
Opponent process theory suggests that the most contrasting color pairs are red–green and blue–yellow.
The black-white color pair is common to all the above theories.
These contradictions stem in part from the fact that traditional color theory has been superseded by empirically-derived modern color theory, and in part from the imprecision of language. For example, blue can be the complement of both yellow and orange because a wide range of hues, from cyan to blue-violet, are called blue in English.
The traditional color wheel model dates to the 18th century and is still used by many artists today. This model designates red, yellow and blue as primary colors with the primary–secondary complementary pairs of red–green, blue-orange, and yellow–purple.
In this traditional scheme, a complementary color pair contains one primary color (yellow, blue or red) and a secondary color (green, purple or orange). The complement of any primary color can be made by combining the two other primary colors. For example, to achieve the complement of yellow (a primary color) one could combine red and blue. The result would be purple, which appears directly across from yellow on the color wheel. Continuing with the color wheel model, one could then combine yellow and purple, which essentially means that all three primary colors would be present at once.