inting. Emphasizing the chromaticity changes across natural daylight phases suggests a warm/cool contrast anchored on red orange. This makes the warmest hue a paint such as pyrrole scarlet (PR255) or pyrrole orange (PO73); the coolest hue is a greenish blue such as phthalocyanine cyan (PB17) or phthalocyanine turquoise (PB16).
Emphasizing the intensity changes in natural light suggests a warm/cool contrast anchored on deep yellow. This makes the warmest hue a paint such as hansa yellow deep (PY65) or quinacridone gold (PO49); the coolest hue is a (dark) blue violet such as ultramarine blue (PB29) or cobalt blue (PB28).
Are there objective criteria for a warm/cool judgment? Yes. All warm surface colors have three reflectance attributes: (1) a sharp transition or "cliff" in reflectance located between the "cyan" to "yellow" wavelengths; (2) maximum reflectance from this cliff to the "red" end of the spectrum; (3) little or no reflectance from the cliff to the "violet" end of the spectrum. This locates warm colors in the hues from light yellow to deep red.
The "cool" hues are defined in nearly complementary terms: (1) a "cliff" profile located between the "cyan" to "yellow" wavelengths, (2) maximum reflectance from this cliff to the "blue" region of the spectrum, and (3) little or no reflectance from the cliff to the "red" end of the spectrum. The "violet" wavelengths must be excluded from the judgment, as these stimulate the a+ opponent function (appear to contain some "red" light). This locates cool colors in the hues from bluish green to deep blue.
Is every color either warm or cool? No. I feel that greens and purples are in themselves neither warm nor cool, because they do not conform to all three of the three criteria just listed for a warm or a cool color, and because they are not salient in the orange to blue transitions of daylight colors.
However, any color can be relatively warmer or cooler in relation to any other color, depending on which color is closer to the arbitrarily chosen warmest (or coolest) color 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 perce