 chromaticity information contributed by equal area 10° cone fundamentals
This divides the spectrum into two parts (above): the monochromatic hues where the S cone outputs do or do not significantly contribute to color discrimination. In long wavelengths (yellows and reds) only the L and M cones can be used to discriminate spectral hues.
color and lightness induction in a "brown" colored tile
from Purves & Lotto (2002)
A background issue is that the maximum chromatic intensity of a surface color is uncoupled from lightness in warm colors, in the sense that increasing the luminance of a warm surface color does not significantly reduce its chroma (as happens in all green, blue, violet and magenta colors). This is because, lacking outputs from the S cones, the L and M cones must provide both luminance (L+M) and chromaticity (LM) information (diagram, right). Adding any short wavelength reflectance increases both the lightness (L+M) and shifts the hue toward white instead of green, because the S cone provides separate chromaticity input.
In this predicament, the visual system apparently imposes a chromatic induction (in comparison to the luminance of a "white" standard) that assigns a different perceptual quality to long wavelength (Y) luminance that is near, or alternately much below, the luminance value expected for a ("warm cliff") reflectance of matching hue.
So experienced "hue" becomes dominated by lightness contrast. However, the reflectance profiles of yellows or reds can become darker valued (the lightness contrast with white is increased) in two different ways:
blacker, meaning that the maximum reflectance in the reflectance profile is reduced well below 100%, even though the transition from minimum to maximum reflectance displays the abrupt "warm cliff" reflectance profile (reduced "vertical" reflectance in the "yellow" and "red" wavelengths).
contrasting sources of luminance and chromaticity in warm vs. cool colors
redder, having maximum reflectance that is close to 100%, but having the "warm cliff" reflectance 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