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 observation is that unsaturated color zones are produced by the lightness contrast that characterizes the related colors of reflecting surfaces. This makes green golds, ochres and browns behave in the same way as grays: they only appear through lightness contrast.
The example at right shows that the same color area can appear brown when viewed with a high luminance contrast (white) background (diagram, right bottom), but will appear yellow orange if the luminance contrast is substantially eliminated by the appearance of shadow (diagram, right top). The reverse is also true: an orange surface can be made to appear brown if the luminance of surrounding color areas is substantially higher. A similar effect appears in yellows and scarlets. The unsaturated color zones are produced by luminance contrast. They are not intrinsic to a specific combination of trichromatic outputs or opponent functions.
What causes this odd color change? The key factor is the span of the S cone sensitivity, which at wavelengths above ~570 nm ("greenish yellow") is less than 0.01% of the peak S cone response (or 1 part in 15,000 of the total chromaticity signal). Compare this to intuitive benchmarks for "invisible" light: in the Stockman & Sharpe cone fundamentals, the S cone sensitivity at 570 nm is less than 2% of its sensitivity at 400 nm, the conventional short wavelength boundary of the visible spectrum, and it is less than 0.05% of the combined sensitivity of the L and M cones at 700 nm, the conventional long wavelength boundary of light.
When a luminance contrast reduces the S cone weighting in the "green" and "blue" wavelengths (because the cones are adapted to respond at the higher luminance of surrounding surfaces), the extremely small S cone signal is pushed below the detection threshold, and our eyes become functionally dichromatic in "yellow" to "red" wavelengths. In effect, brown signals a contrast induced colorblindness (tritanopia) in normal vision.
cone mixture curves across spectral hues
curves show the proportion of the total