 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, producing familiar changes in the appearance of our environment. Hues appear to shift warm or cool, colors become more or less saturated, lighter or darker, and complementary shadow colors change from violet to blue green.
Blackbody Color. The obvious next step is to find a method that can describe or define the relative amounts of yellow or blue bias in a "white" light. A simple way to do this, for natural light and most "white" artificial lights, is by the light's blackbody temperature.
In 1900 the Austrian physicist Max Planck mathematically described the spectral power distribution that would be produced at different temperatures by a perfectly radiating object, called a "black body" because no light would reflect from it. (This work led to the development of quantum mechanics, which won Planck the Nobel Prize in 1918.)
These blackbody curves approximately match the spectra radiated by many natural light sources, including heated metals, electrical discharges and stars. In all these cases, an entire spectral emission curve can be specified by its blackbody temperature alone.
blackbody spectral emittance curves
for temperatures of 2860°K, 5000°K and 6500°K, corresponding to CIE illuminants A, D50 and D65; as the blackbody temperature increases, the peak emittance increases and shifts from infrared to ultraviolet wavelengths
Shown above are the blackbody curves corresponding to three standard "white" illuminants published by the CIE. They illustrate the three characteristics of all blackbody radiation: (1) a continuous, smooth curve with a single peak emittance; (2) a shift of the peak emittance as temperature increases, from a peak in the far infrared (7200 nm) for a blackbody at room temperature to a peak in ultraviolet (30 nm) for a blackbody at 100,000°K (degrees Kelvin); and (3) an enormous increase in total radiant flux as temperature increases.
color changes in daylight
(top) noon; (bottom) late afternoon; note change in the sky color
color changes in daylight
(top) morning; (bottom) midday diurnal color 