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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 black 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.

Hal
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  • http://www.youtube.com/watch?v=qyYA3Znvz1w You can find explanation here –  Jul 01 '15 at 19:24
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    I just noticed this statement: "White light contains every colour". That is not correct. For example, there is no brown in white light, yet our brain considers it a colour. Brown is a red with some green and blue mixed in. – hdhondt Apr 16 '17 at 06:11

2 Answers2

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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.

hdhondt
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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 '14 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 '14 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 '14 at 09:36
  • 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 '14 at 16:21
  • If you mix paints together you should get black. No, because paints are pigments. Each non-black pigment particle in the mix reflects some fraction of light. The only way the paint could look black (i.e., reflect none of the light) is if all of the particles are black. – Solomon Slow Jul 01 '15 at 20:43
  • @james large: So you're telling me that if I mix red, green and blue paints the resulting colour will be white??? Try it with some real paints. – hdhondt Jul 03 '15 at 07:12
  • I did not say "white". White is highly subjective. http://web.mit.edu/persci/people/adelson/checkershadow_illusion.html If you mix red, green, and blue pigments, then the mixture will reflect some red light, some green light, and some blue light. Only a third of the particles will be red, so the mixture will reflect only about a third as much red light as the pure red pigment. Likewise for green and for blue. If you had a card that reflected one third of all light, and you held it up next to another card that reflected, say 95%, you would call the first one "gray." – Solomon Slow Jul 03 '15 at 09:46
  • @SolomonSlow one thing tell to create additive color we need absence of white light or absence of sunlight? – S. M. Feb 10 '22 at 02:07
  • @hdhondt one thing tell to create additive color we need absence of white light or absence of sunlight? – S. M. Feb 10 '22 at 02:08
  • @hdhondt, SolomonSlow "so with any device there are colours we can see but which the device cannot produce. "----what does mean this statement? Any device which produce the color that we see, but you are saying we see the color which cannot be produced. Didn't understand properly. Please explain little. – S. M. Feb 10 '22 at 02:44
  • @User4567 Any device has a range of colours it can produce. The eye has a range of colours it can see. Those ranges overlap, but not completely. See, for example, https://www.deprintedbox.com/blog/wp-content/uploads/2014/03/color-space-diagram.jpg – hdhondt Feb 10 '22 at 03:42
  • @User4567 For additive colour you use 3 colours: red, green, and blue. In equal quantities they make white (or grey); in unequal amounts they mix to make the other colours. Look at a TV screen with a magnifier, you'll see nothing but red, green and blue dots. White light or sunlight are not needed - but too much of it will wash out the screen. – hdhondt Feb 10 '22 at 03:45
  • @hdhondt that understand. But you wrote in answer any device which has color we can see but device can't produce? My question is how? – S. M. Feb 10 '22 at 03:49
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    Look at the graph in my link. The outside "triangle" shows the colours the eye can see; the inner triangles show the range of colours various devices can produce. On those devices you will never see the colours outside the device's triangle, as they cannot be produced. – hdhondt Feb 10 '22 at 04:39
  • @hdhondt if RGB provides glow, why we need cmyk for printing? I mean why cmyk is efficient for printing than RGB? – S. M. Feb 10 '22 at 16:34
  • @User4567 because the RGB pixels in a monitor produce light, while the CMYK pixels in a print remove light from the illuminating source (e.g. sunlight). – hdhondt Feb 11 '22 at 01:08
  • @hdhondt when white light incident on cyan ink it absorbs red light and reflects green and blue—remember then green + blue light = cyan. My question is it only reflects green and blue light among 7 colors (white light) ? Remaining lights is absorbed like red color? – S. M. Feb 11 '22 at 01:36
  • That is essentially correct. – hdhondt Feb 11 '22 at 05:18
  • @hdhondt suppose incoming light that enters the into any red object then except red everything get absorbed. Then this red light which I see is refraction or reflection light? I seem it's refraction? What is right? – S. M. Feb 12 '22 at 12:11
  • Refraction is when a light ray is bent (as when going through a lens). Reflection is when a light ray bounces off a surface. If you see red light going through glass, that means glass absorbs the other colours. You should really start new questions, instead of using comments to ask additional ones. – hdhondt Feb 13 '22 at 05:46
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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

KvdLingen
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