optimal color stimulus (right) shows how closely intense red, orange, yellow and yellow green pigments match the ideal and therefore can reach a chroma nearly equal to the chroma of monochromatic lights.
The key feature here, in awkward language, is that warm colors can retain saturation sideways increasing the width of the wavelengths of maximum reflectance does not lower the saturation of the color. No other surface hues have this property.
Warm and Cool in Paints. Now consider the effect this warm cliff must have on the lightness of different warm hues. As we read these reflectance curves from right to left, it is as if a luminosity curtain were being drawn back from a window of light, revealing more and more reflectance as the apparent hue shifts from deep red to orange to yellow. There is a close relationship between lightness and hue for intense colors across the warm color span.
We can see this clearly if we plot the measured lightness of commercial watercolor paints against their hue angle in the CIELAB color space. (Use this chart to identify the pigments located at a specific CIELAB hue angle.)
masstone lightness and hue angle
CIELAB L* and h measured in 600 commercial watercolor paints displayed at maximum chroma
The line of yellow colored points on the left spans the warm colors from hue angle 20 (quinacridone pyrrolidone, PR N/A) to hue angle 95 (cadmium lemon, PY35). (The dark blue points above this curve at left are lighter valued rose pigments; those below the curve are earth pigments and dull synthetic organic pigments such as perylene maroon.) This curve peaks at a light or lemon yellow, because the lightness of light yellow paints is around 95 and white paints are at 98. Increasing the reflectance further quickly causes yellow to desaturate into white as the warm cliff crosses into the darker "blue" and "violet" wavelengths.
The chart also shows a reverse relationship between lightness and chroma in the most intense cool colors, shown in pale blue from approximately hue angle 200 (cobalt teal blue, 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 conta