Warm and Cool Split

Real light does this by itself. Sunlight is warm and the sky filling the shadows is cool, so a picture built this way reads as observed rather than invented.

It also lets a scheme hold six or seven colours without falling apart, because the organising idea is temperature rather than hue. That is more freedom than any wheel relationship gives.

Decide the temperature of the light, then make the shadows the opposite. Warm key with cool fill is the usual pairing, and the reverse works for moonlight.

Let the division follow the light rather than the objects. A red jacket in shadow goes cool. Colouring by object is the mistake that makes this look like a filter.

Allow several hues on each side. This is the difference between a temperature split and a complementary pair: warm can be amber, coral and gold at once.

Use it for anything lit: a portrait, an interior, a sunset exterior, a film frame, a product shot, a scene at a window.

Leave it alone for flat graphic work. There is no light in a Bauhaus poster for the temperature to follow.

Complementary pairs two hues. This pairs two temperatures, which can carry many hues on each side and so reads as light rather than as design.

Teal and orange grading is this scheme reduced to exactly two colours and applied after the fact. This is the general case.

Rembrandt lit almost everything this way, with a warm key and the cool of the room filling the rest. Modern film grading rediscovered it as teal and orange, which is the same observation with the subtlety removed.

The shadows come back neutral grey, which is the default and kills the scheme. Say the shadows are cool blue-grey and never neutral.

The colour follows the objects rather than the light. Say the division follows the light, so an object in shadow takes the shadow temperature whatever colour it is.