he 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 elsewhere, complementary color effects that involve green vs. purple should be qualitatively different from the contrasts around warm vs. cool colors.
unsaturated color zones
A major feature of the warm/cool color contrast is that certain kinds of lightness or chroma contrast cause perceptually unique colors to appear among reds and yellows but not among blues, greens or purples. These new colors are brown, ochre and green gold.
They cannot be produced by manipulating the brightness or colorfulness of an isolated red, orange or yellow color area; they only appear in the contrast between related colors. Yet they appear perceptually very different from the saturated hues: brown looks nothing like orange, and green gold nothing like yellow.
I've called these unsaturated color zones. (There doesn't seem to be a standard name for them; some texts refer to them as "grays".) The chart below suggests the color variety they produce, in comparison to two shades of green.
unsaturated color zones
warm colors at optimal lightness (L) and chroma (C, middle row), with reductions in lightness (top three rows) or in chroma (bottom three rows) to 80%, 65% or 50% of the optimal values; focal locations of green gold, ochre and brown are outlined
In fact, as shown above, these colors can be produced by reducing either the lightness or chroma of the pure hue, although lightness has a greater impact than chroma across all hues, and lightness contrast is most important and chroma contrast least important in light yellow hues.
Boundary of Unsaturated Color Zones. The unsaturated color zones only appear within a limited span of the color wheel, roughly centered on the warm end of the warm/cool contrast at a CIECAM hue angle of about 35°. This splits the color wheel into two parts: the relatively small area between red and yellow where the unsaturated color zones appear, and the much larger circuit of violets, blues and greens where they do not.
The diagram shows the approximate extent of these browns, ochres and green golds as a proportion of the