Color Temperature
Warm colours seem to come towards you and cool ones seem to fall back. It is not a physical property of the light — it is a habit of human vision reliable enough that painters built depth on it for five centuries.
Where the word comes from
Two unrelated ideas share the name, and they are inverted with respect to each other. The physicist's colour temperature is the kelvin value of a blackbody radiator: a candle at around 1800 K, tungsten at 2800 K, midday daylight at 5500 K and an overcast sky far higher. On that scale the bluest light has the highest temperature, which is the exact opposite of the studio's meaning of the word. The painter's warm and cool came first, from firelight and sunlight against water and shadow, and it has never surrendered the term.
In the studio
Leonardo wrote down the observation that gives the technique its name in Italian art theory: distant objects lose contrast and shift towards blue, because there is more air between them and the eye. Aerial perspective turns temperature into a measuring instrument — the mountain is far away because it is cool and pale, not because it has been drawn smaller. Venetian painters ran the same trick at close range, laying cool half-tones against warm lights so that a face turns in space without a single hard outline.
Why it matters
Temperature is relative, and that is the part that takes years to see. There is no warm red and cool red in the abstract: a red is cool because the red beside it is warmer. A painter working with three reds is not choosing hues, they are choosing a sequence, and the whole illusion of a form turning away from the light depends on differences small enough that a photograph often loses them.
Today
The physicist's meaning took over the technology. Film stocks were balanced for tungsten or daylight and gave you a blue or orange cast if you used the wrong one; digital cameras replaced that with a white-balance setting that does the correction arithmetically, which is why an image can be "corrected" into looking like nothing at all. Screens are calibrated to a white point around 6500 K, and every warm interface theme is a decision to move away from it.
Pigments used: Ultramarine · Vermilion · Green earth
Essays: Painting with light · When colour arrived at the cinema
See also: Leonardo da Vinci · Titian · Technicolor Cinematography