The distinction between warm and cool colors, not understood until the late 18th century, is based on perceived temperature. Those colors with a warm bias, evoke images of sun and fire, and generally include the hues red through yellow (plus browns and tans), while those with a cool bias are associated with an overcast day and include hues from blue-green through blue-violet (plus most grays). Blacks may have either a warm or cool bias.
Warm colors tend to advance into a composition and to stimulate and arouse viewers, while cool colors recede and tend to relax. Additionally, because objects in the distance are perceived to have a cool, bluish tint, juxtaposing warm and cool colors on a two-dimensional surface can help create the illusion of three-dimensional space. The perception of scale is also affected by color temperature: cooler objects appear smaller compared to those of equal scale with a warm bias.
Color mixing may shift the underlying bias of the combined color depending on the colors used. For example, primary yellow, a warm color, can be shifted to a cool lemon yellow by adding white, and primary blue, a cool color, can be shifted to a warm green with the addition of certain yellows.