ctance transition shifted far into the long wavelengths where the L and M cone sensitivity is much weaker (reduced "horizontal" reflectance in the "yellow" to "red" wavelengths). This is revealed by the way the spectrum locus dives toward the achromatic center (becomes darker) along a constant hue angle or black limit at around CIECAM hue angle of 33°; diagram, right.)
The combined effects of decreased hue sensitivity and the ambiguous interpretation of darkened color (increasing blackness or redness) means there is a disproportionately large number of metamers among dark, dull, red or yellow surface colors a third perceptual justification for unsaturated color zones.
How does color vision represent these ambiguities? Just as it does in tritanopia by anchoring the hue sensation in the opponent balance between red and green (the r/g opponent function).
Recall that, at high reflectance with a "warm cliff" profile, a roughly equal mixture of red and green produces the perception of yellow. Yellow is the perceptual token for a surface color that approximately balances the r/b function at high surface reflectance. Any yellow is a signal of both high chroma and high reflectance in warm surface colors, which means the color is near or above the red boundary shown above. Increasing the lightness and/or chroma of any color from yellow green through scarlet takes it toward its affinity hue (diagram, below) and produces a clear perception of the proportion of yellow hue in the color. Adding any S cone response shifts the color toward white (or green), so whiteness or greenness becomes the warm color signal for short wavelength reflectance yellow and blue really do make green.
the r/g balance in unsaturated warm colors
For darkened colors below the red boundary, as chroma and lightness decrease, the "yellow" sensation weakens and then disappears. It is no longer possible to describe the color in relation to yellow, except intellectually (by studying color vision). In this case the color appears as a brown or ochre that seems to lean either toward red or green. This red/green balance signals indirectly the relationship of the color to the saturated affinity hues:
If the color appears to contain more red than green, then it is at a hue angle below 60° (see diagram, above right), and would if made brighter shift toward a red (orange or scarlet) color.
red > green orange or scarlet
If the color appears more green than red, then it is located somewhere at a hue angle above 60°, and would if made brighter shift toward a yellow or greenish yellow color.
red < green medium yellow or green
If the color appears neither red nor green, then it is at a hue angle of about 60° and would shift toward a deep yellow or light orange if made brighter.
red = green deep yellow
"Made brighter" means brightened by lightness contrast, that is, at a higher lightness and higher chroma. The effect of lightness contrast is to increase lightness proportionately more in greenish (yellowish) unsaturated colors, but to increase chroma proportionately more in reddish unsaturated colors. This is because a pure red orange is much darker valued, and also more intense, than a pure yellow.
The perceptual quality of brown as a red/green mixture is the same as that of green as a blue/yellow mixture. Greens appear either cool (bluish) or warm (yellowish), and similarly browns, ochres, umbers and siennas can appear either cool (greenish) or warm (reddish). The difference is that a balanced green and blue mixture appears as a dull green, but a balanced red and green mixture appears as a brown or umber.
painting warm & cool
In this section I emphasize the key points argued in this long and somewhat speculative page, and offer some guidance on manipulating the warm/cool contrast in painting.
I don't provide a separate discussion for manipulating colors in digital or video media, because these are created by light projection systems and additive light mixtures, not illuminated surface colors and subtractive pigment mixtures.
What is the origin of warm/cool contrast? The perceptu