ing," "attention getting," "cheerful" or "arousing"!
Most of the warm/cool color effects do not appear in color comparisons made with single wavelength or prismatic colors. For example, "violet" monochromatic light has the highest saturation of any hue and appears extraordinarily "advancing" and "arousing" in comparison to "yellow" light, which is bland and dull. Just the reverse is true in paints.
Because cool colored paints are typically darker and less intense than their warm complements, they make effective background colors for warm colors. The warm colors capture our attention and seem to stand out from the cool background; like the setting for a jewel, the background provides an enhancing contrast. But these are jewels of chroma and lightness, not hue. The same arousing, advancing contrast appears in the last fiery orange leaves that remain in a tree of autumnal brown even though the red orange and brown are both warm colors and are in fact the same hue.
the origin of warm/cool
By assigning warm color depth and mood effects to lightness and chroma, it might seem that the warm/cool contrast is illusory, just a crude way to summarize the lightness and saturation differences between different hues.
But that's the wrong conclusion. The depth and mood effects were grafted onto the warm/cool contrast by late 18th and 19th century "color theorists"; they were not part of the contrast as it was applied in Baroque landscape painting. So we still have the fourth question to answer: why is the warm/cool contrast fundamental to our visual response to color and to the manipulation of colors in painting?
The first step to an answer is that the warm/cool contrast originates in diurnal or climatic changes in illumination, specifically as seen in landscape settings. Thus the Oxford English Dictionary describes 18th century usage to include:
Cold - applied to tints or colouring which suggest a cold sunless day, or the colder effect of evening; esp. to blue and grey, and tints akin to these.
Warm - suggestive of warmth, said especially of red or yellow ... to become 'warmer' or more ruddy: "On a bright morning of July, when the grey of the sky was just beginning to warm with the rising day".
These and similar 18th century sources, unpolluted by later color theory misconceptions, show clearly that the warm vs. cool contrast originated in observations of the changing illumination produced by sun and sky. Specifically:
changes in the sun's altitude during the day cause the color of daylight to shift from a cool (bluish) tint at high noon to a warm (yellowish to reddish) tint after sunrise or before sunset
changes in the sun's declination cause seasonal changes in the sun's maximum elevation above the horizon, causing the average illumination and temperature to increase from winter to summer
the atmosphere produces changes in illumination intensity and color through the filtering effects of smoke, dust, water vapor and clouds.
The examples (right) illustrate the changes in natural light from dawn through midday to late afternoon. (These changes are best observed through the window of a darkened room, as Monet did when he painted his series of Rouen Cathedral facades.) The color changes are clearly from warm (yellow) to cool (blue). As the sun declines in the sky, the light dims and the sky color shifts from deep blue to cerulean; in surface colors, reds and yellows become more saturated, yellow greens become warmer and lighter valued, and blues or blue greens become grayer and darker.
color changes across daylight phases
by Hiroshi Yoshida
The Japanese artist Hiroshi Yoshida made a fine set of three woodblock prints illustrating these color effects (above).
These changes physically occur because the daylight spectral power distribution contains different proportions of "red", "yellow" and "blue" wavelengths at different times of the day, different seasons of the year and different geographical locations, and under different atmospheric conditions. These variations in the color of the illumination mix subtractively with surface colors, p