 between short and long wavelengths of light. So the warm/cool contrast is fundamentally a judgment of the balance in a light mixture between the wavelengths shorter than green (blue and violet) and longer than green (yellow and red).
Reflectance Criteria for "Warm" Colors. The next point is whether there are consistent attributes in the reflectance of surface colors perceived as warm or cool. There are, in particular for "warm" colors. These are apparent in the difference in color perception between lights and surfaces.
For all parts of the emitted spectrum that is, for colors of lights the relative saturation increases as the wavelengths of light are limited to a narrower band of color. At the extreme, a spike of single wavelength (monochromatic) light is the most saturated color stimulus physically possible for any hue at any brightness. And this is true no matter whether the hue is red, orange, yellow, green or blue.
Despite this, we find in artistic practice the consensus that only lights in the hue range from red through yellow to white are considered warm. A green or blue light, by itself or as the source of illumination in an architectural space, is not identified as warm and may even be termed cool. So although chroma or saturation may explain the arousing or advancing effect of warm colors, it does not by itself define an essential warm or cool color attribute. The same reasoning applies to the brightness or lightness of a color.
However, for reflected colors the colors of the real world the perception of chroma is more complex. As we saw in the discussion of optimal color stimuli, the maximum possible chroma for a surface color is defined by its lightness or total reflectivity, which means as colors become lighter they necessarily become duller. So the general rule for hue purity in surface colors should be: high saturation signals high reflectance within a limited part of the spectrum, although this means intense surfaces will also be relatively dark surfaces; pastel (light valued) colors typically have muted saturation.
But for warm surface colors even this rule does not apply: warm surface colors retain chroma across increasing lightness. This is apparent in the diagram of MacAdam limits: purple, blue or green hues contract toward the white point (become less saturated) as lightness increases, but for hues from yellow green through red the maximum chroma boundaries remain at maximum saturation, along the spectrum locus although the range of hues gradually contracts toward yellow.
The reason for this unique quality is what I call the "warm cliff" reflectance curve that is characteristic of all saturated red, orange and yellow paints. Invariably, all intense warm hues have three reflectance attributes: (1) a distinct "cliff", or abrupt increase in the reflectance curve between "cyan" and "orange", (2) consistently high (90% or above) reflectance on "red" side of the cliff, and (3) consistently low reflectance (20% or less) on the "blue" side of the cliff. This locates warm colors in the hues from deep red to light yellow (approximately CIELAB hue angles 30° to 90°)
the "warm cliff" reflectance curve
(a sampling of intense red to yellow pigments (left to right): hansa yellow light PY3, hansa yellow deep PY65, perinone orange PO43, naphthol red deep PR170)
This sampling of reflectance curves shows how consistent this "warm cliff" is across all highly saturated warm color pigments, from lemon yellow (hansa yellow light, PY3) to deep red (naphthol red deep, PR170). These curves roughly bracket the wavelengths where there is a "plateau" in the photopic light sensitivity curve, which means that luminances in this range will appear brighter than equal radiances at higher or lower wavelengths.
daylight spectral power distribution and blackbody curve at 5500°K
D55 curve from
If we examine the average chroma of different watercolor pigments (in the artists' color wheel), the highest available chroma appears in red to red orange hues. Comparison of these reflectance curves with the idealized warm profile for a matching 