Complementary
Colours opposite each other on the wheel. Put them side by side and each looks more intense; mix them together and both die. The same relationship does the two most useful and opposite jobs in painting.
Where the word comes from
The pairing has a physiological basis: stare at a red shape and look away, and the afterimage is green. Goethe wrote about the phenomenon at length in 1810 against Newton, and Michel Eugène Chevreul gave it a working law in 1839. Nineteenth-century painters read them, in translation and in summary, and the wheel entered the studio as an instrument rather than a curiosity.
In the studio
Delacroix studied the pairs obsessively and left notebooks full of them. Van Gogh made the most explicit statement of the method in a letter about The Night Café, where he says he tried to express the terrible passions of humanity by means of red and green — the interior is built on that opposition and it is deliberately unpleasant. Monet used the softer version outdoors: shadows on snow painted blue-violet against warm sunlight, which critics initially read as an eye disorder rather than an observation.
Why it matters
The unglamorous use is more important than the vibrating one. Complementaries neutralise each other, so the way to darken or dull a colour without killing it is to add a little of its opposite instead of black — a grey mixed from vermilion and viridian keeps a life that a grey mixed from black and white never has. Most of the sober, beautiful neutrals in Old Master paintings are complementary mixtures, not greys out of a tube.
Today
The side-by-side version is the one that survives in design advice, usually badly. Two complementary colours at equal value and full saturation produce a shimmering edge that is genuinely tiring to look at, which is why the pairing works as accent-on-field — one small saturated area against a large muted one — and fails as a fifty-fifty split.
Pigments used: Vermilion · Viridian · Ultramarine
Essays: How colour became a science · When colour arrived at the cinema
See also: Eugène Delacroix · Vincent van Gogh · Claude Monet