Complementary colors

Two colours are complementary when their hues are half a turn apart. Because they are as far from each other as the wheel allows, no other pairing separates them further, which is what makes the combination read as deliberate rather than accidental: one colour has nowhere further to go from the other.

Which pairs those are depends entirely on whose wheel is being used, and this is the part that causes arguments. On the red, yellow, blue wheel taught in art classes, red pairs with green, blue with orange, and yellow with purple. On the additive wheel that screens actually work in, red pairs with cyan, green with magenta, and blue with yellow. A perceptual wheel such as the one oklch describes gives a third set of numbers again. All three are correct within their own model, so “the complement of red” is only a well posed question once you say which wheel you mean.

Complementary pairs are loud on purpose, and they misbehave when used in equal measure. Set two fully saturated complements at the same lightness side by side and the shared edge appears to shimmer, an effect of the eye rather than the screen, and each colour makes the other look more intense than it is. The conventional fix is to refuse the symmetry: give one hue most of the area and the other a small fraction of it, and separate them in lightness as well as in hue. Softer relatives exist for when the full pair is too much, notably the split complementary (one colour plus the two hues either side of its opposite) and the triad.

In interfaces the pairing is usually a job rather than a scheme: the brand hue carries the surface and the navigation, and its complement is spent on exactly one thing, the sale badge or the primary action, precisely because nothing else on the screen can compete with it. Two cautions. Hue opposition provides no lightness difference at all, so a complement never satisfies a text contrast requirement on its own. And red with green, the most famous pair of all, is the one that the most common colour vision deficiencies flatten into a single muddy hue.