ve 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 optimal color stimulus (right) shows how closely intense red, orange, yellow and yellow green pigments match the ideal and therefore can reach a chroma nearly equal to the chroma of monochromatic lights.
The key feature here, in awkward language, is that warm colors can retain saturation sideways increasing the width of the wavelengths of maximum reflectance does not lower the saturation of the color. No other surface hues have this property.
Warm and Cool in Paints. Now consider the effect this warm cliff must have on the lightness of different warm hues. As we read these reflectance curves from right to left, it is as if a luminosity curtain were being drawn back from a window of light, revealing more and more reflectance as the apparent hue shifts from deep red to orange to yellow. There is a close relationship between lightness and hue for intense colors across the warm color span.
We can see this clearly if we plot the measured lightness of commercial watercolor paints against their hue angle in the CIELAB color space. (Use this chart to identify the pigments located at a specific CIELAB hue angle.)
masstone lightness and hue angle
CIELAB L* and h measured in 600 commercial watercolor paints displayed at maximum chroma
The line of yellow colored points on the left spans the warm colors from hue angle 20 (quinacridone pyrrolidone, PR N/A) to hue angle 95 (cadmium lemon, PY35). (The dark blue points above this curve at left are lighter valued rose pigments; those below the curve are earth pigments and dull synthetic organic pigments such as perylene maroon.) This curve peaks at a light or lemon yellow, because the lightness of light yellow paints is around 95 and white paints are at 98. Increasing the reflectance further quickly causes yellow to desaturate into white as the warm cliff crosses into the darker "blue" and "violet" wavelengths.
The chart also shows a reverse relationship between lightness and chroma in the most intense cool colors, shown in pale blue from approximately hue angle 200 (cobalt teal b