ervation is that unsaturated color zones are produced by the lightness contrast that characterizes the related colors of reflecting surfaces. This makes green golds, ochres and browns behave in the same way as grays: they only appear through lightness contrast.
The example at right shows that the same color area can appear brown when viewed with a high luminance contrast (white) background (diagram, right bottom), but will appear yellow orange if the luminance contrast is substantially eliminated by the appearance of shadow (diagram, right top). The reverse is also true: an orange surface can be made to appear brown if the luminance of surrounding color areas is substantially higher. A similar effect appears in yellows and scarlets. The unsaturated color zones are produced by luminance contrast. They are not intrinsic to a specific combination of trichromatic outputs or opponent functions.
What causes this odd color change? The key factor is the span of the S cone sensitivity, which at wavelengths above ~570 nm ("greenish yellow") is less than 0.01% of the peak S cone response (or 1 part in 15,000 of the total chromaticity signal). Compare this to intuitive benchmarks for "invisible" light: in the Stockman & Sharpe cone fundamentals, the S cone sensitivity at 570 nm is less than 2% of its sensitivity at 400 nm, the conventional short wavelength boundary of the visible spectrum, and it is less than 0.05% of the combined sensitivity of the L and M cones at 700 nm, the conventional long wavelength boundary of light.
When a luminance contrast reduces the S cone weighting in the "green" and "blue" wavelengths (because the cones are adapted to respond at the higher luminance of surrounding surfaces), the extremely small S cone signal is pushed below the detection threshold, and our eyes become functionally dichromatic in "yellow" to "red" wavelengths. In effect, brown signals a contrast induced colorblindness (tritanopia) in normal vision.
cone mixture curves across spectral hues
curves show the proportion of the total 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" refle