e as the cool complement. These choices define a yellow/blue contrast as an alternative axis of warm/cool effects. It makes all hues containing yellow (from orange red through yellow green) warm colors, and all hues containing blue (from blue violet through blue green) cool colors.
This warm/cool choice is a proxy for the long and short wavelength analysis optimal colors identified in my synthesis of surface colors. Optimal orange is the color produced by the sum of all long wavelength light (above 570 nm), and is closely matched by nickel dioxine yellow (PY153) or isoindolinone yellow (PY110). Optimal blue is the color produced by the sum of all short wavelength light (below 485 nm), and it is closely matched by ultramarine blue (PB29).
This approach is especially suitable when the artist intends to use yellow as the symbol of light. This matches our experience of color variations in three ways. First, the hue contrast is aligned with the largest value contrast the lightest saturated hues are yellow, and the darkest hues are blue violet. Second, as explained below, the major visual axis that captures chromaticity variations in natural light (landscape light) extends from yellow to blue. Third, most colors shift toward yellow as the luminance on or through them becomes more intense: blue violets become blue, blues become blue green, greens become yellow green, oranges become deep yellow, and "spectrum" reds become orange. (Purples shift either toward red or toward blue, depending on the balance of the color.) The effects of reflected, filtered or shadowed light transition in the opposite direction, from yellow to red or green blue to blue. Thus, layers of translucent yellow material redden as they become thicker, cyan tropical shallows darken to blue as waters get deeper.
J.M.W. Turner provides some classic examples of this approach. In many of his paintings, the lightness of colors is closely aligned with the color temperature (yellow is the lightest color, blue violet or gray the darkest), so lightness and warmth go hand in hand. Jan Vermeer's paintings also show a career shift in color design, from a red/green contrast in his earlier paintings to a more austere, atmospheric yellow/blue contrast in his later works. The palette by Lucy Willis is a good example among contemporary watercolorists.
However, the fact that artists define and use the warm/cool contrast in different ways shows that there is no such thing as a universally "warmest" color or "coolest" color, either. Color attributes such as lightness, hue or chroma are inherently defined by context in particular, by the chromaticity of the light illuminating the objects in the image so the best choice of warm or cool colors will depend on the design goals or painting purposes (abstract, expressionist, representational) for which the colors are used.
Is Every Color either Warm or Cool? The second question is whether every color should be described as either warm or cool, or only reds and yellows as opposed to greens and blues.
There are two conventions: one assigns an absolute quality to a color (this color is always warm) or a relative quality to a color (this color is warm in comparison with that color).
Most color theory texts categorize greens and purples as either warm or cool, because color theory likes things tidy. But I suggest that painters consider the hues from green yellow to green, and from violet blue to violet red, as fundamentally neither warm nor cool. In these hues, blue and red, or yellow and green, visibly mix they are warm/cool hybrids. The complementary contrast between green and violet also lacks the stridency of the contrast between red and green or yellow and violet blue. This yields the labeling scheme shown below.
warm vs. cool colors in the color circle
Whatever your views on this fundamental dichotomy, it's common practice and often useful to describe colors as relatively warmer or cooler in comparison with another color. This simply indicates whether the color is closer to red orange (warmer) or to green blue (cooler) than the compari