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I'm trying to reconcile these facts,

  • If I had paints of the primary colours and I mixed them together, I would produce gray.
  • White light contains every colour.

My most convincing attempt to answer the question: The colours of objects that don't produce light are the colours those objects don't absorb. A white object doesn't absorb any colour, a blue object absorbs all colours but blue, a green object absorbs all colours but green, etc. So by mixing paints of the primary colours I mix influences that collectively absorb all colours and reflect all colours; whereas, if I had paint that only absorbed all colours I would have black paint, and if I had paint that only reflected all colours I would have had white paint. Gray is a combination of white and gray, so I found this answer intuitively satisfying. However, I don't know if it's an accurate explanation of the observation.

Why does mixing every paint colour produce gray instead of white?

Thank you.

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2 Answers 2

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You are confusing additive and subtractive colour mixing. If you mix paints together you should get black, not white.

In additive mixing (as used in TVs and monitors), you create light, which is then mixed. When you mix the three primary colours (red, green and blue), you produce white. Other mixes produce other colours, for example red and green combine to produce yellow.

When you use paints, you are using an external light source (the sun or a light bulb) and each paint reflects some of the wavelengths and absorbs others. For example, yellow paint absorbs the blue wavelengths, leaving red and green, which mix to yellow. This is called subtractive mixing, and the primaries are cyan, magenta and yellow; when you mix paints of these colours, the result is black. Adding additional colours to this mix keeps the result black, as there is no more light to reflect. Other colours are made up by mixing the primaries.

With both additive and subtractive mixing, the result of mixing colours depends on the purity of the primaries. No paints are "perfect" cyan, magenta or yellow, and as a result the mix will not be completely black. You may get a dark brown or purple, depending on the paints you use. This is one (of several) reasons why printers use black as well as CMY.

The same goes for monitors: you never get "pure white" - which is typically defined as light with a colour temperature of 5500K, about the same as sunlight. Some monitors can be set for different temperatures. Some are set to 9000K, giving white a bluish cast. Interestingly, the colours that can be displayed on a monitor do not match those of a printer (or paint). A monitor can display colours that a printer cannot print, and vice versa. Every device has its own colour gamut, usually smaller than the eye's gamut, so with any device there are colours we can see but which the device cannot produce.

The reason why all this mixing occurs is because our retina has sensors for red, green and blue, and the brain mixes these inputs to tell us what colour we are seeing. This is why the primaries are RGB, or CMY.

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Combinations of dyes will yield a subtractive color mix, since dye molecules absorb light of some wavelengths while passing others. If one has dye with particles which absorb everything but green, adding particles which absorb everything but red won't help any light get through. Paint, however, often has particles which reflect certain wavelengths while absorbing others. If one has paint with particles that only reflect green, adding particles that reflect red will allow some red light which would have been absorbed by a green particle... –  supercat Feb 17 at 19:54
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...to get reflected by a red particle first. Since some light that reflects off a red particle will hit (and be absorbed by) a green particle before bouncing clear of the paint, the behavior of mixed paints isn't exactly additive, but it isn't subtractive either. Note that many paints include dyes, which do behave in subtractive fashion; two kinds of "blue" paints may appear identical, but yield very different results when combined with a "yellow" paint. Some kinds of yellow, mixed with true "blue" (not cyan) will yield green--not consistent with additive or subtractive color rules. –  supercat Feb 17 at 19:58
    
And then there are metallic paints. There is a huge chasm between colour theory and actual colour work. Hopefully we've given @Hal the incentive to investigate further. –  hdhondt Feb 18 at 9:36
    
Simplified color theory works nicely when when combining lights to achieve a certain look, or when combining dyes whose absorption spectra are largely non-overlapping, so as to achieve a certain look under some particular illuminating spectrum. The way materials' colors interact, though, depends upon their behavior at individual wavelengths. I wonder how hard it would be to construct a telescope-like device which would clearly show the spectral content of a spot at the center of the view field, marked with a cross-hairs or other indicator. That could assist understanding of "real" color. –  supercat Feb 18 at 16:21

You are right. The coloured paint absorbs part of the light colours. So the reflected light is more gray than white. Remember that the primary colours of light are red, green and blue, whereas the primary colours of paint are yellow, Magenta and cyan. The latter usually incorrectly referred to as red and blue. Mixing Magenta, cyan and yellow should absorb most of the light, returning black, but it does not do true black. That's why there is a black cartridge in your color printer

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