lue, PG50) to hue angle 290 (ultramarine blue, PB29). The saturated blue hues lose lightness as the hue shifts from "blue green" toward "violet".
This is obvious if we plot the chroma of these paints in relation to their hue, which shows that as warm pigments lighten (from red to yellow) and as cool pigments darken (from turquoise to violet), they both increase in chroma. But the range of chroma is greater, and the maximum lightness of chroma is much higher, for warm colors in comparison to all other hues. This contrasting relationship between lightness and chroma is partly why the warm hues have qualitatively opposite effects in color experience.
masstone chroma and hue angle
CIELAB C and h measured in 600 commercial watercolor paints displayed at maximum chroma
If we just multiply the lightness of a color by its chroma, the highest values are in the warm colors around CIELAB hue angle 30 (roughly from pyrrole red, PR254 to pyrrole orange, PO73), which includes many brands of cadmium scarlet. In those hues specifically, the edge of the reflectance cliff is right on the wavelength of maximum sensitivity for the L cones, while its base is at the wavelength of maximum sensitivity for the M cones. Throughout this hue range, for maximally intense colors, there is no significant output from the S cones.
theoretical and actual red orange reflectance curves
optimal color in white;
Reflectance Criteria for "Cool" Colors. By examining these pigment plots, and partly by reversing the "warm" color reflectance attributes, I can suggest that all cool hues have three reflectance attributes: (1) a "cliff" in the reflectance profile located between "cyan" and "yellow"; (2) high or maximum reflectance from this cliff into the "blue" end of the spectrum; and (3) minimum or no reflectance from this cliff to the "red" end of the spectrum. The only qualification is that the "violet" wavelengths (below ~460 nm) must be excluded from the judgment, as these stimulate the r+ opponent function (as shown here) and therefore appear to contain 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 t