 lightness of "pure" (most saturated) colors in a typical artists' color wheel, which puts the pure pigments at the circumference. Color markers are indicated by pigment color index name. The boundary of the unsaturated color zones defines an involute starting at the neutral tone for a magenta, increasing rapidly in relative lightness through the reds and red oranges, and finally intersecting the most saturated yellow green color at a hue angle of around 110°. The approximate location of focal brown, ochre and green gold are shown for reference.
unsaturated color zones in a generic color wheel
boundary of unsaturated colors as a proportion of the lightness of pure pigment color, at hues anchored by common pigments
The boundary is difficult to summarize in terms of chroma or saturation changes. The gist is that:
Mixing the pure color with white never produces these new colors; it only produces a pastel or tint of the color and typically shifts the hue as well (toward yellow for oranges or deep yellows, toward violet for reds and bluish reds).
Mixing the pure color with black ("lightness boundary", above) always produces these new colors, as it reduces both lightness and chroma; the impact of added black is strongest in light yellows (where a 5% reduction in lightness will shift the pure color into green) and weakest in bluish reds.
Mixing the pure color with a gray of equal lightness ("chroma boundary", above) typically produces these new colors, as it reduces chroma only; the effect is strongest in hues from yellow to scarlet. Added gray does not produce a categorical change among light yellows, as these already have a lightness close to white.
An interior boundary between the unsaturated colors with a reddish or greenish appearance crosses underneath all yellows and turns toward the achromatic center of the color wheel at around a hue angle of 60°, approximately the location of isoindolinone yellow (PY110) or nickel dioxine yellow (PY153). Colors near this green boundary have a greenish appearance (for example, raw umber or green gold) will usually shift back to a yellow appearance if sufficiently diluted or mixed with white.
A variety of color labels have been attached to various shades or tones within the unsaturated color zones in the CIECAM aCbC plane. This table provides a guide.
As with any color, the label applies to a category or cluster of similar colors that are centered on a focal color with a specific lightness, chroma and hue. Just lightness or chroma by itself does not adequately locate the color changes. For example, an extensive study by Bartleson put the focal color of "brown" at around Munsell 5YR 3/6, which corresponds to a CIECAM JCh of about 25, 32 and 52°, respectively very near the location of burnt umber (PBr7).
The difficulty for artists who want to mix an unsaturated warm color is that they must recognize a specific brown not simply as an orange hue, but an orange of a specific lightness and chroma. Because hue is very hard to identify in unsaturated warm colors, this results in a lot of fruitless mixing back and forth to create a specific dull warm hue from more saturated paints a problem that is most commonly encountered when mixing skin tones.
What Causes Unsaturated Color Zones? The subjective qualities of the unsaturated color zones are barely mentioned in the color vision literature. Joy Turner Luke explains the "strange case of yellow and brown" this way: "Apparently when the combined response from the L and M (Y) cones is below a certain level, perception is more affected by the signal from the L-M channel than the signal from the Y-B channel."
This offhand explanation doesn't do the job. Why do green golds, ochres, siennas, umbers and maroons only appear in related color judgments, if the r/g and y/b opponent functions also apply to the perception of unrelated light colors? And why does the signal from the "L-M channel" affect only the appearance of reds and yellows, and not symmetrically the appearance of greens, when the y/b ("Y") response is "below a certain level"?
The starting obs