ption 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 illumination? Absolutely. Brilliant, balanced natural light occurs under a high sun and a clear sky. All hues are rendered perfectly, and both the colorfulness contrasts and lightness contrasts are optimal. All directly illuminated surface colors appear in their unbiased local color and can be painted in that way.
In daylight shadows are tinted with a desaturated red blue (such as indanthrone blue, PB60 or a grayed cobalt blue, PB28), not a purple. A general yellowing of color or a slightly increased saturation from orange to yellow green and a decreased saturation from magenta through cyan can be used for expressive emphasis or to show variations in illuminance intensity produced by the angle of surfaces to the light source ("brighter" illuminance = yellower or, for warm hues, more saturated color).
Dim light either means greatly reduced illuminance or filtered (chromatically altered) light. If the light is dim, the chromatic balance among colors, measured in the spectral composition of the reflected light, is roughly the same. However, it produces very different mood effects than bright light of the same chromaticity. Dim light seems softer, more intimate and pleasant if it has a "warm" chromaticity, but seems cold, gloomy and depressing if it has a "cool" chromaticity. In either case the average value may be darker, but values always contract toward the average value (extreme lights and darks disappear, with the larger shift toward gray occurring in the dark values).
If the light is filtered, the color depends on the tinting layer that produces the filtering. Overcast clouds produce a darkening that is approximately the same chromaticity as unfiltered daylight, or slightly bluer. In painting this is commonly represented as a shift toward blue, with a loss of colorfulness (graying) and contrast in all colors, but especially warm colors (which shift toward green).
Near sunset, short or "blue" wavelengths are strongly filtered from sunlight by atmospheric dust and smoke, causing the "blue" component in all colors to 