Color Temperature
Whether a color reads as warm (red/orange/yellow side) or cool (blue/cyan/green side) of the spectrum.
Definition
Color temperature describes the perceived warmth or coolness of a color. Warm colors (reds, oranges, yellows) suggest sun, fire, energy; cool colors (blues, cyans, greens) suggest water, sky, calm. The split runs roughly through the green-magenta axis on the color wheel. In design, warm palettes feel inviting and active; cool palettes feel professional and calm. The CIE measures actual color temperature in Kelvin (lower = warmer light).
Formula
On the HSL color wheel: warm = hue 0-60° or 300-360°. Cool = hue 180-270°. The bands at 60-180° (yellow-green to teal) and 270-300° (purple) are 'transitional'.
Example
Red (0°) and orange (30°) are warm. Blue (240°) and cyan (180°) are cool. Mauve (276°) sits in the cool/transitional band.