Colour Harmony
A relationship between the colours in an image that the viewer perceives as coherent rather than accidental.
In depth
The classical schemes — complementary, analogous, triadic, split-complementary — are descriptions of arrangements that tend to work, not rules. Harmony in photography is more often achieved by removal than by construction: eliminating one intrusive colour from a frame usually does more than adding a considered palette.
Examples in practice
Waiting for a red car to leave the frame resolves a landscape whose greens and blues were otherwise harmonious.