Colour Mixing

There seems to be some confusion about what the primary and secondary colours are. Well, it depends.

For the purpose of art, the primaries are
red, yellow, blue;
this is called the RYB colour model. The secondaries are then
green = yellow & blue;
purple = red & blue;
orange = red & yellow.
(This is a subtractive colour model.)

Two models arise in science (in physics and in the science of perception).

There is the additive RGB model of light (adding to black), with primaries
red, green, blue;
secondaries
cyan = green + blue;
magenta = red + blue;
yellow = red + green;
and finally
white = red + green + blue.

There is the subtractive CMY model of pigment (subtracting from white), with primaries
cyan, magenta, yellow;
secondaries
red = magenta × yellow;
green = cyan × yellow;
blue = cyan × magenta;
(then, tertiaries, including purple and orange);
and finally
black = cyan × magenta × yellow.

The confusion may come from the fact that RYB has two primaries of in common with RGB, yet the former is subtractive whilst the latter is additive.

Really, RYB has more in common with CMY. RYB might be thought of as an inaccurate theory of pigments, only approximating CMY in the sense that RYB has a smaller gamut than CMY (the gamut of RYB is a strict subset of that of CMY).

There is a useful correspondence between the CMY and RGB models: that of opposites:
blackwhite;
redcyan;
greenmagenta;
blueyellow.

After all this, it is worth mentioning that colours are not real, in the sense that they are a perceptual construct rather than physical.

The various types of colour receptor in our eyes (usually three types) have response distributions to overlapping, but fairly narrow bands of the electromagnetic spectrum.

It is possible for different spectra to be perceived as the same (for example, a single frequency of ‘yellow’ light will appear identical to some combinations of ‘red’ and ‘green’ light). That they are different might be perceived indirectly by their being absorbed differently by the pigmentation of a particular material. (Another example is that it is possible for a yellow object to look black in yellow light, because the yellow pigmentation might be a mix of ‘red’ and ‘green’ under a light that is ‘yellow’; this might happen for example with a yellow car under a yellow sodium street lamp.)

Some people perceive the world differently due to differences in their colour receptors, perhaps due to so-called colour blindness, or conversely with slightly enhanced colour perception due to differentiation of receptors due to a genetic mutation. Some animals, of course, perceive the world differently. We normally have only 3-dimensional colour vision. Some animals can perceive finer spectral differences with a greater number types of receptor; some animals can even perceive frequencies (into the infrared or ultraviolet) or aspects of light (such as polarisation) that we can’t.

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