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Mixing of different wavelengths of light results in white, but why is that when paint with different colors are mix results in black?

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    $\begingroup$ Are you familiar with the additive / subtractive color distinction? $\endgroup$ – Alfred Centauri Sep 13 '18 at 0:40
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    $\begingroup$ Just to add to existing answers (not an answer itself) - this is precisely why monitors that produce light use RGB palette while print uses CMY(K). CMY is actually subtracting RGB - if you subtract from white light Red, Green or Blue you get respectively Yellow, Magenta or Cyan. So you're also operating on the same principle of applying the change to only one colour respective to one of the cone types in our eyes at a time. Contrast is added as there are always some wavelengths deflected even if you mix all 3 paints. Thus adding a black to the pallete helps to resolve that (to some level). $\endgroup$ – Ister Sep 13 '18 at 7:55
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    $\begingroup$ You can't get black by mixing different colored paints. Paint gets its color from opaque particles of pigment. All of the light reflected by a paint spot is reflected by the top-layer of particles. If you mix red paint and blue paint, the red particles in the top layer will reflect red light, and the blue particles will reflect blue light. But the only way you can get black would be if none of the pigment particles reflected any light. You can't get black by mixing different colors of light. $\endgroup$ – user205719 Sep 13 '18 at 15:58
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    $\begingroup$ Ink is different. You can get black by mixing different colors of ink. That's because ink doesn't reflect light, it transmits light. It's the paper underneath that reflects the light, and the ink colors the reflected light by absorbing some of it. The more different inks you add to an ink spot, the more light is absorbed. And if it absorbs enough light, then it looks black. $\endgroup$ – user205719 Sep 13 '18 at 16:00
  • $\begingroup$ If the answers you get here do not give you enough information, you could try Google. Googling the title of your question yields a treasure trove of sites answering this question in various ways. $\endgroup$ – Cort Ammon - Reinstate Monica Sep 13 '18 at 23:46
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Mixing light does result in white, but the black mixture of paint happens due to how paint works. Paint has color not because it's emitting light, but because it's absorbing colors other than the one that's supposed to be the paint's color. As such, when you mix paints, they absorb more and more of the spectrum, resulting in black.

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    $\begingroup$ What is not immediately obvious, is that one paint reflects one color, another reflects another, so shouldn't a mix of paints reflect all colors, resulting in white? $\endgroup$ – LLlAMnYP Sep 13 '18 at 10:25
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    $\begingroup$ @LLlAMnYP Good point, and it's almost what happens. If you could plot a spectrum of the reflected light, it would be almost perfectly balanced. But the thing is, so much energy is absorbed by the paint that it barely reflects enough power at all. A very dim white light is just gray (i.e. black). $\endgroup$ – Gabriel Golfetti Sep 13 '18 at 10:33
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    $\begingroup$ You can get both. If you are mixing paints, then what one doesn't aborb, another one does (so they just take away incrementally from the same light until nothing is left). But if you put pixels of different colors side by side, you get halftone printing, and you see average colour, which is the same hue, but brighter because none of the dots absorbs it all. However one has to be careful about mixing and black colour: the subtractive mixing model breaks down at high concentrations - it's no longer linear, and you usually get brownish tone. $\endgroup$ – orion Sep 13 '18 at 13:23
  • $\begingroup$ "Mixing light does result in white, but this happens due to how paint works." - how does the way paint works affect mixing of light? Is there a typo there? $\endgroup$ – npostavs Sep 13 '18 at 21:15
  • $\begingroup$ @npostavs oops, right. Not really a typo, just poor sentence structure. $\endgroup$ – Gabriel Golfetti Sep 13 '18 at 21:31
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Mixing light is additive since you are adding electromagnetic waves with different wavelengths together.

The color of materials as paint relies on a different principle. Only certain wavelengths of the impinging light get scattered back and the rest gets absorbed. For example, red paint only scatters back the red wavelengths and absorbs the others. (This also means if you light the red paint with blue light, you will not see much). Now if you mix all colors of paint, the whole (visible) spectrum of the impinging light will get absorbed. Hence, paint is subtractive. (This is also the reason why black paint gets warmer than white paint; the absorbed light is converted into heat energy).

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It actually has nothing to do with light itself. An easy way to demonstrate this is to get some panes of differently colored glass, and line up a light source to shine through multiple panes. You'll notice the color gets remarkably darker the more differently-colored panes you use. Shining a light through a red pane, a green pane, and a blue pane won't get you white; it'll get you pretty close to black.

It's not light vs paint; it's light addition versus light subtraction.

When you see 'green paint', it's not because the paint is adding green light. It's because its subtracting out non-green light. Same thing with a pane of green tinted glass - it's not adding green light, it's subtracting out non-green.

Compare that to a computer monitor. If you've got a bunch of green pixels lit up, lighting up the neighboring red pixels doesn't subtract from the amount of green light; it simply adds additional red light to the equation - which your eye then perceives as a shade of yellow.

So, what happens when you mix a bunch of colors of paint? It actually doesn't become black - it just becomes a grayish mix matching the average brightness of the colors. Try it - mix Sky Blue, Canary Yellow, and Pink - you won't get a color anywhere close to black; but if you mix Navy Blue, Brown, and Forrest Green, you'll get something a lot darker. An easy way to mentally picture what you'll see is if you imagine painting a checker-like pattern out of the colors you're mixing, and then walk far enough away that the colors all blur together.

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