1. The warmth of colour as measured on the Kelvin scale, based on the fact that a blue flame burns hotter than a red flame. Warm or reddish colours are counter-intuitively relatively cool (around 2 000 K) while cold or bluish colours are relatively warm (around 5 400 K). In photography, a technical way of identifying colour differences based on the temperature at which an inert substance glows a particular colour.
2. The relative warmth or coolness subjectively ascribed to particular hues. Warm colours typically include red, yellow, and orange. These tend to be perceived as salient: as ‘advancing’ figure (see figure and ground). This is generally assisted by high saturation and greater luminance. Cool colours typically include blue, green, and purple. These tend to be perceived as receding or as ground. This is generally assisted by low saturation and lower luminance. This dimension of colour perception applies quite widely cross-culturally.