Triadic color scheme

Pick a hue, add 120 degrees, add 120 more. That is the whole rule, and it is one of the oldest members of the colour harmony family: a set chosen by geometry on the colour wheel rather than by taste. Because the three are as far from each other as three hues can get, none of them reads as a variation of another. A triad is the maximally contrasting three-colour set, in the same way that complementary colours are the maximally contrasting pair.

That is also the problem. Three hues at equal saturation and equal lightness have equal claim on the eye, and a screen painted from them argues with itself: nothing looks primary because everything is. The working version of the rule is one hue dominant and the other two as accents, which is what the 60-30-10 rule formalises. In practice that means the triad supplies hues, not amounts: the lead hue is spent as a large quiet surface or a dark header, and the other two appear at chip and button scale, usually as tints and shades rather than at full strength.

The same geometry generalises. Split-complementary keeps the base and replaces its opposite with the two hues either side of it, which keeps the opposition and loses the vibration. Tetradic, also called the square scheme, takes four hues 90 degrees apart, and is the same idea one step further and one step harder to balance. All of them are spacing rules, and none of them says anything about how much of each colour to use.

Two cautions before trusting the arithmetic. Which wheel you are counting on matters: the painter’s red-yellow-blue wheel puts a triad at red, yellow and blue, while a hue angle in CSS counts on an RGB wheel where the same three steps land somewhere else, and OKLCH counts on a third wheel again. And equal spacing is not equal presence, because the sRGB gamut holds much more chroma at some hues than others; a triad mixed at one chroma will still have a loud member and a quiet one. Adjust by eye afterwards, and treat the 120 degrees as the starting point rather than the answer.