in some "red" light. This locates cool colors in the hues from bluish green to deep blue (approximately CIELAB hue angles 180° to 270°).
This implies that, for cool hues, the cone response profile is always S > M > L. (Note that, for surface warm hues, the reverse is not always true, because L and M can be added together without substantially reducing the chroma of the color, whereas a blue green formed by S+M is always duller than a blue from S alone.) In cool hues, adding L output acts subtractively on the M output and thereby reduces the saturation of the color. In other words, as the S output increases, the color effect of L+M changes from luminance increasing to chroma reducing (as explained below).
Warm/Cool in Color Space. Finally, we can compare the actual chroma of modern artists' pigments with the gamut of optimal colors, which have the maximum physically possible chroma of any surface color that does not fluoresce. The diagram shows this comparison on the CIECAM aCbC plane.
artists' pigments and optimal color limits on the CIECAM aCbC plane
gamut. This suggests that our perception of chroma and saturation is adapted to, or habituated within, the limits of physically possible surface stimuli even though these stimuli are essentially theoretical or "ideal" and are not found in nature.
The optimal color limits may be closest to the pigment limits along the orange/blue direction of the color space because this is the direction of variation in daylight phases. It is interesting that two ridges in the optimal color surface roughly correspond to the warm/cool contrast (red orange to blue green). Since surface colors appear as the subtractive mixture of the surface and illumination spectral profiles, changes in light chromaticity will have the strongest effect on colors in this direction. It is unclear how much of this structure develops in response to the naturally occurring variations in daylight and how much is "hard wired" in the opponent functions.
Our perception of surface chroma does not correspond to the maximum saturation possible in blue or purple hues of monochromatic light. However, extremely intense blue or purple substances may be hard to form for basic chemical reasons, and would therefore not appear in the domain of natural colors. The pigment examples probably capture the range of color experience produced by physical materials under natural light.
What about Purple? Finally, all the evidence discussed above suggests that violet is neither warm nor cool. I argued above that green is neither warm nor cool because the gamut of daylight spectra contains no green, and the same is true for violet or purple in all blackbody curves no matter how intense a blackbody radiator, it never achieves a color beyond blue violet.
As the green/purple dimension is perpendicular to the warm/cool contrast (defined on red orange), these hue seem unrelated to color temperature. In fact, a magenta/green contrast seems to be associated with different levels of brightness adaptation, and is therefore related to changes in the illuminance, not changes in the light's spectral profile.
Exclusion arguments can also be based on reflectance curves. The reflectance curves of blue green and blue pigments do not conform to any of the three criteria characteristic of warm hues a warm cliff reflectance profile, maximum reflectance on "red" side of the cliff, and no reflectance on the "blue" side. Rather, they have subdued "hump or bump" profiles, maximum reflectance in the short wavelengths, and no reflectance in the "red" wavelengths. This makes them visual complements to the warm colors. In contrast, the reflectance curves of violet pigments and green pigments show some of these warm color attributes but not others. So those hues do not clearly belong on either side of the warm/cool contrast.
In paints, then, warm colors are limited to the range from deep red to light yellow. If we consider only colors that violate all three criteria, then cool colors are limited to the range from blue green to deep blue. This means, as argued elsewher