y influence our color judgments, and in my view lightness and saturation explain the depth and mood effects associated with warm or cool colors. In fact, Alfred Munsell noted the "advancing" effect of light valued and saturated colors and developed principles of color harmony around them.
The proof is that when the lightness and/or chroma of warm colors are made less than the lightness and/or chroma of their cool complements, those special warm color depth and mood effects either disappear or shift over to the cool colors!
warm color effects caused by lightness and/or chroma
cool colors can easily appear "advancing" or "arousing" if they are lighter and/or more intense than the warm colors around them; a white disk appears "closer" than a black disk
But what makes lightness and saturation "attention getting" or "advancing"? The luminance: they are the most luminous or chromatic objects in the visual field, and our visual system perceives extreme brightness or chroma as similar.
Vision continually isolates contrast or novelty wherever it appears, as a primitive strategy to identify unusual (attention getting) features in the environment and bring them (advancing) to our attention. Even in an urban consumer environment, intensely saturated colors are infrequent in our routine visual experience. We occupy a fairly gray world, colorimetrically speaking: saturated colors stand out because they are unusual.
I think the conventional wisdom about warm colors evolved primarily from comparisons between pure pigment paints. In general, warm colored pigments achieve a higher chroma and lighter value than their cool complementary equivalents. (Ultramarine blue is a spectacular exception.) A saturated yellow or orange paint is much lighter valued and more intense than any green or blue paint. But if the yellow or orange is mixed with black so that its lightness and chroma are matched to a complementary iron blue, the color is transformed into a raw umber or burnt umber. And I have never heard anyone claim that brown is "advancing," "attention getting," "cheerful" or "arousing"!
Most of the warm/cool color effects do not appear in color comparisons made with single wavelength or prismatic colors. For example, "violet" monochromatic light has the highest saturation of any hue and appears extraordinarily "advancing" and "arousing" in comparison to "yellow" light, which is bland and dull. Just the reverse is true in paints.
Because cool colored paints are typically darker and less intense than their warm complements, they make effective background colors for warm colors. The warm colors capture our attention and seem to stand out from the cool background; like the setting for a jewel, the background provides an enhancing contrast. But these are jewels of chroma and lightness, not hue. The same arousing, advancing contrast appears in the last fiery orange leaves that remain in a tree of autumnal brown even though the red orange and brown are both warm colors and are in fact the same hue.
the origin of warm/cool
By assigning warm color depth and mood effects to lightness and chroma, it might seem that the warm/cool contrast is illusory, just a crude way to summarize the lightness and saturation differences between different hues.
But that's the wrong conclusion. The depth and mood effects were grafted onto the warm/cool contrast by late 18th and 19th century "color theorists"; they were not part of the contrast as it was applied in Baroque landscape painting. So we still have the fourth question to answer: why is the warm/cool contrast fundamental to our visual response to color and to the manipulation of colors in painting?
The first step to an answer is that the warm/cool contrast originates in diurnal or climatic changes in illumination, specifically as seen in landscape settings. Thus the Oxford English Dictionary describes 18th century usage to include:
Cold - applied to tints or colouring which suggest a cold sunless day, or the colder effect of evening; esp. to blue and grey, and tints akin to these.
Warm - suggestive of warmth, said