diurnal changes in the yellow/blue balance of daylight. The r/g opponent function provides adjustments in the relative balance of the L and M contribution to the Y component of the y/b function, which restores the perception of "pure white" illumination as light shifts farther into the "red" wavelengths.
In the diagram above, the arrow indicating the direction of "yellow" wavelengths (around 575 nm) shows that the y/b opponent function can easily compensate for the daylight shifts in chromaticity down to about 5000°K. But under extreme "blue" or "red" (late afternoon) daylight phases the r/g function is also involved. This is easier to see if the chromaticities of daylight CCTs (disregarding their typically low saturation and huge differences in luminance) are shown as hue angles on the CIECAM chromaticity plane.
color analogs to daylight spectra chromaticities
the hue of blackbody temperatures illustrated as spectral locations on the CIECAM a*b* plane
This diagram allows comparison of the color shifts in natural light with the traditionally defined warm/cool contrast: the match is quite good. It also suggests the relative contributions of the r/g and y/b opponent functions in chromatic adaptation. The y/b function makes the major adjustments around the average solar "white", while the r/g function tracks hue changes at lower temperatures. In video production, there are analogous Y/B and R/Y controls to adjust the image white balance; digital artists use green/magenta, red/cyan and yellow/blue controls that change the balance between the three complementary contrasts that define the secondary color wheel.
Warm hues (matched by CCTs below 5000°K) are smeared across the hue range from yellow to red; cadmium pigments represent this range very well. The daylight CCTs above 6500°K start at a teal blue and shift to a middle blue at 10,000°K; this range is represented by cobalt pigments.
The color of the clear sky (skylight) varies substantially by geographic latitude, altitude, season, humidity, distance from the zenith, time of day and concentration of atmospheric ice, dust or smoke. The distribution of chromaticities is again roughly parallel to the blackbody locus, but the average sky chromaticity is usually above a CCT of 10,000°K, corresponding to a dominant wavelength of about 470 to 475 nm (CIELUV hue angle of about 235° to 245°). The best paint matches to the typical blue sky color are a dulled cobalt blue (PB28) or iron blue (PB27), both visual complements of orange yellow.
The luminance of sky colors vary widely. As an upper bound, cumulus clouds that appear "pure white" have an albedo (reflectance) of around 70%-80%, equivalent to a light gray; it is the luminance contrast with the surrounding darker sky that makes them appear white. The sky typically appears relatively dark, with a luminance near the zenith that approximately matches a middle gray (reflectance 30%). Again, moderately diluted cobalt paints reproduce these values well.
Color Rendering Index. The orientation of the lines of constant CCT (above) shows that artificial lights with the same CCT can have a definite yellow, green, blue or purple tint, even though rated as "white" lights. This ambiguity is introduced in two ways: (1) by defining CCTs as metameric matches between a blackbody curve and a light, which can ignore radical irregularities in the spectral curve of the light, and (2) by defining CCTs as the nearest blackbody chromaticity to the chromaticity of the light, even when there is a visible color difference between them. Lights with the same CCT and even the same "white" chromaticity can have very different spectral emission curves, which will produce different appearances in surface colors.
These differences represent the color rendering quality of a light. The color rendering index (CRI) is a numerical rating of how closely the color appearance of surfaces viewed under the light matches the color appearance of the same surfaces viewed under a blackbody light of the same CCT. It is calculated by averaging the colorimetric differences (if an