From:
<http://www.av8n.com/imaging/dye-spectra.htm>
We can summarize this by the qualitative equations
yellow plus yellow makes orange (3)
orange plus orange makes red (4)
These observations are wildly inconsistent with the "color theory" that you
learned in third grade. There is no way to explain them in terms of the
"color wheel" or anything like that. However, facts are facts. These
observations are entirely real; no tricks or optical illusions are involved.
Mixing colored dyes is commonly called subtractive color mixing, but
properly it should be called multiplicative color mixing instead, for the
following reason: Suppose you know a certain amount of ink attenuates a
certain wavelength by a factor of X. If you have twice as much ink, the
attenuation will be X2, not 2X.
----------------------------------------------------------------------------
---
A few comments:
1. The standard color wheel effectively only uses one color coordinate - the
hue angle. That's why you can't describe changes in saturation or value
using it. You can describe all color changes using a color system with 3
color coordinates, the L*a*b* or xyY system, for example.
2. In additive color mixing we change the amount of red, green, or blue
light from 0 to 100%.
R: 0-100%
G: 0-100%
B: 0-100%
Starting at 100%R, 100%G, and 100%B light (white), we can subtract these
colors by various amounts of subtractive colors to also (approximately)
generate the color space obtained using lights.
For example, to obtain 25%R and 75%G, you can do it with lights
Additive: 25%R light and 75%G light
or start with white paper and use
75% cyan colorant on top of 25% magenta colorant.
In subtractive color mixing using the primary subtractive colors of cyan,
magenta, and yellow as used in standard terminology, you don't use more than
100% cyan, magenta or yellow. If you do, that's not considered subtractive
color mixing.
3. You can of course increase dye or ink concentrations above that used,
say, in color ink jet printers. Then you use the Beer-Lambert Law to figure
out what's going on, as done by JD.
Finally, water is a common example of a "dye" that changes color because it
is a weak and selective absorber. When the path length of light in water is
small, water is transparent. When the path length of light in water is
large, water preferentially absorbs red and green light, so appears blue.