Colors on a TV or computer monitor screen are additive: Adding
different colors of light together produces a brighter color. The
primary colors of light which most closely match human color
receptors are red, green, and blue. Mixing equal amounts of
red and green light together additively gives brown or yellow,
depending on the total brightness of the red and green light.
Opaque pigments in paint work by selectively reflecting the light
that strikes them, which is a subtractive process. A thick layer of
paint or many thin layers make it more likely that incident light
will strike a pigment particle and be reflected back out (when the
color of the light matches the color of the pigment). Subtracting
different colors of light from the incident light produces a color
that is roughly the average brightness of the different pigments.
The primary colors of paint pigments are red, yellow, and blue,
which can be mixed together to give the widest gamut of colors
for human color receptors. Mixing equal amounts of yellow and
blue paint pigments together subtractively gives green, with a
brightness roughly equal to the average brightness of the yellow
and blue pigments, since the pigment particles are essentially
side-by-side. Yellow is inherently lighter than blue in human
vision, so the more yellow and less blue in the mixture, the
lighter the resulting green. Mixing equal amounts of red and
green together subtractively gives a brown which is an average
of the included red, yellow, and blue pigment brightnesses.
Colored inks, dyes, watercolor paints, or wax crayons on a white
background work mainly by selective transmission, which is also
subtractive, but is different from how opaque paint pigments
work, so that I call the process "transparent" color mixing rather
than "subtractive" color mixing. Different colors of light travel
through the transparent pigments, are reflected by the white
background, and travel back through the pigments again to reach
the viewer. The thicker the layer or layers of ink or dye, the darker
the color tends to be, since less light is transmitted. Mixing
different transparent colors gives a resulting color darker than
any of the component colors of the mixture. The primary colors
of transparent pigments are yellow, cyan (or "process blue"), and
magenta (or "process red"), which, like the primary subtractive
colors of opaque pigments, are the three colors which give the
widest gamut of colors for human color receptors. And like the
subtractive colors of opaque pigments, yellow plus process blue
gives green, and process red plus green gives brown. The brown
will be darker than any of the individual transparent pigments.
The attached images depict these three methods of color mixing:
RGB is for light, RYB is for opaque pigments, and YCM is for
transparent pigments.
-- Jeff, in Minneapolis
