or around the hue circle. These judgments must be made in context with the other colors in the image or scene, including the scene illumination, and not as abstract comparisons between color wheel locations.
Are warm colors advancing or arousing? No. The illusion of relative depth or a specific mood effect are not consistent attributes of warm or cool hues. Colors typically "advance" or come forward in an image because they are (1) light valued and/or (2) highly saturated. These confusions developed because warm pigments as a group are lighter valued and more saturated than cool pigments.
Mood effects depend on combinations of both lightness and chroma: strongly saturated hues at medium lightness are usually "arousing" or "vibrant"; highly saturated hues at high lightness are usually "cheerful"; low saturation hues at high lightness are typically "restful"; and low saturation hues at low lightness are "somber" or "subdued". These effects are mostly independent of hue, but are strongly dependent on the image, usage or scene context. (Audrey Hepburn is not somber and subdued in those slim black pants!)
What are the unsaturated color zones? These are relatively dark (nonreflecting), low chroma colors perceived as categorically different from light valued and high chroma colors of the same hue. They only occur among "warm" hues, and include maroon (dark red), brown (dark orange), ochre (dull deep yellow), raw umber (dark deep yellow) and green gold (dull or dark light yellow).
What causes the unsaturated color zones? In warm surface colors, color appearance is strongly affected by the lightness contrast between a color area and surrounding surfaces.
This seems to be a compensation for the lack of S cone response (similar to tritanopia) to long wavelength light. Because there is no third cone sensitivity across the "yellow" or "red" wavelengths, the surface color chromaticity (the ratio between L and M outputs) is not significantly altered by increased luminance (L+M outputs), coupling chromaticity and lightness perception in yellow to red surface colors.
However, because of the remarkable consistency of the "warm cliff" reflectance profile in all yellow to red material colors, the visual system is able to compensate for this deficiency. It compares a color's lightness (L+M) to the lightness expected for a matching optimal color of the same hue, which represents the physical limit for a "pure" color of the same hue (LM). If the contrast lightness is near this maximum for a given LM balance, perception signals this fact by adding a distinct "yellow" color sensation to the color appearance.
A warm color can only exceed this optimal contrast ratio through the addition of short wavelength reflectance, which both increases the color lightness (L+M response), which preserves the yellow sensation but mixes it with a chromaticity shift toward white or green (separate S response). (In other words, L+M and S really do make green!)
When the color's lightness falls substantially below the lightness expected of a maximally saturated color of the same hue, the yellow sensation disappears and the color can only be characterized tritanopically, as a balance between red and green sensations. These yellowless mixtures of red and green create the unsaturated color zones.
Saturated blue reds (such as the quinacridones) that reflect some "blue" wavelengths make a dark violet when mixed with black paint or illuminated by a broadband blue or near UV light, while a "spectrum" red given the same treatment turns deep brown or black. Browns and ochres do not appear in surface colors illuminated by a broadband orange or red light source, because this provides no S stimulating "green" or "blue" light and therefore no ambiguity in the "warm" part of the spectrum. Brown, ochre or green gold colors appear in any darkened color where the reflectance almost entirely stimulates the L and M cones and the S contribution is small enough (in relation to quantity of S stimulation in the light) to be ambiguous.
Does the apparent warm/cool contrast depend on the illu