 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 perceptual importance of the warm/cool contrast probably arises in the capability of the human visual system to adjust to changes in the color and intensity of natural illumination under different phases of daylight. In response to these illumination changes, color vision changes both its relative sensitivity to light (chromatic adaptation) and its relative judgment of colors (color constancy) to maintain a consistent appearance in surface colors.
A heightened awareness of the warm/cool contrast in painting technique probably dates from the representation of diurnal and climatic changes of light in late 17th century landscape paintings, and was explicitly extended in artistic practice during the 18th century.
What are these changes in natural light? Natural daylight changes in two ways: brightness and chromaticity (color). Under clear skies, brightness ranges from a high around noon to a low just after sunset. The chromaticity changes, as defined by the correlated color temperature, range from a greenish white at noon to an intense scarlet at sunset (for direct sunlight), or from a cerulean or greenish blue at noon to a deep yellow at sunset (for sunlight and skylight combined).
Light intensity also has specific warm/cool effects as it interacts with different kinds of materials. In general, as the intensity (illuminance) of light on or through materials increases, the apparent color of the materials shifts toward yellow: blues become blue green, greens become yellow green, oranges become yellow, reds become orange; violets shift either toward red or toward blue, depending on the hue balance. The effects of reflected, filtered or shadowed light change in the opposite direction, from yellow to red or green blue to blue. (For example: layers of translucent yellow material redden as they become thicker, greenish pools darken to blue green as waters get deeper, etc.)
What are the warmest (coolest) hues? There is no such thing as a "warmest" or "coolest" hue in colorimetry. Two traditions have developed in representational pa