Warm and Cool Colours
The perceptual division of the spectrum into warm reds, oranges and yellows and cool blues, greens and violets.
In depth
The division has real perceptual consequences: warm colours appear to advance toward the viewer and cool ones to recede, so a warm subject against a cool background gains apparent depth. The association also carries emotional weight, which is why the same scene graded warm reads as nostalgic and graded cool reads as clinical.
Examples in practice
Warming a portrait's skin while cooling the background pushes the subject forward without any change to sharpness or contrast.