– What happens when an artist mixes paint for a painting? Are the pigments actually changing physically/chemically? What is actually happening to make the paints change color?
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There’s two basic ways to mix colour; additive and masking.
Additive (not sure if that’s the correct word) increases colours like the way a colour tv or monitor does. Masking partially masks colours by mixing in other shades and black like the way a colour printer does.
There’s no chemical change, just a difference in the way light bounces off the paint and the way our eyes perceive it
The colors you see in paint are due to pigments reflecting certain wavelengths of light. Adding different pigments changes the color, most of the time by adding the reflection of different wavelengths of light. However, black absorbs all wavelengths of light so adding black pigment will cause the paint to appear darker because the black pigment is absorbing light rather than reflecting it. No actual chemical changes happen in most cases.
Edit: clarification
The pigment particles in the paint are ground incredibly fine. When two paints are mixed, the two different pigment particles are mixed together and so closely that our eyes can’t distinguish one from the other but perceive a new color, of them mixed.
Picture if you had a million red pingpong balls, and a million yellow pingpong balls, and you mixed them together in a giant pool and looked at them from miles above. Your eyes couldn’t pick out single balls, it would look orange.
Lets say you mix some red and yellow oil paint together to make an orange paint, and put it under a microscope. You would be able to see individual red and yellow pigment particles, but the smear of paint on the slide would look orange.
The pigment molecules dont change at all, they are just mixed together.
Take a look at the pixels on your screen; if you look real close, you can see that each pixel is made of a seperate red light, green light and blue light. Those lights are kind of like the pigment molecules, they’re both just really small and really close together.
Just want to say, I read this and initially thought, “What a dumb question, obviously they…they…. huh.” And started scratching my head. And I realized I had no idea and got curious myself. It was fun to read the answers and learn about it!
There are some excellent examples here of how paint particles look under a microscope which may make sense when you see them. The National Gallery has produced this one about works by [Monet](https://www.nationalgallery.org.uk/media/15524/roy2007.pdf)
Everyone else has touched on the ~~additive~~ E: subtractive color mixing idea, but I want to address this:
> Are the pigments actually changing physically/chemically?
*Generally* not, at least not nowadays, because a lot of research has gone into finding pigments that are good at being pigments – vibrantly colored, as opaque as possible to hide the color of the thing the paint is on top of, etc. – but also *don’t react with each other*, so that they continue contributing the color you expect them to no matter what random combination you mix together.
That hasn’t always been the case. For example, most paints used to include lead(II) carbonate for basically the same reason we put titanium dioxide in paints today – it’s very opaque and a very pure white, which helps the paint to cover up what’s behind it without affecting the color of the paint itself all that much. But lead(II) carbonate reacts with hydrogen sulfide, which is constantly being produced and released into the air in tiny quantities by basically living things due to breakdown of proteins. Lead(II) carbonate + hydrogen sulfide makes lead(II) sulfide, which is this icky dark brown/blackish color instead of white – causing lead(II) carbonate paints to discolor over time.
A certain orange-red lead oxide usually called “red lead” or minium similarly discolors and turns black when it forms lead sulfide, and it can even do this when mixed with other sulfide-containing pigments like vermillion (mercury sulfide) or orpiment (arsenic sulfide). Azurite is an unstable blue form of copper(II) carbonate which degrades over time to a dark brownish-green – a combo of a stabler, green form of copper(II) carbonate + black copper oxide.
There are many more such reactions involving old timey pigments. You may find [this paper](https://heritagesciencejournal.springeropen.com/counter/pdf/10.1186/s40494-017-0125-6.pdf) to be of interest.
It’s partially for this reason – and partially because they’re super expensive, and partially because they’re super poisonous – that many of the pigments discussed in that paper have been phased out of use, and replaced with cheaper, less toxic, more vibrant, and *unreactive* pigments today, like copper phthalocyanine and iron oxide.
Nope, they don’t react or change at all (except maybe in some rare cases with exotic pigments), they’re just mixing together very closely. Pigment particles are small, and your eye can only tell individual things apart if they’re above a certain size and distance from each other.
LCD screens are a good example of that, although light and paint mix in sightly different ways. Unless you’re an eagle, you probably can’t see red, green, and blue dots all over your screen, but that’s all you’re looking at. Combining those three colors together is enough to make (almost) every color your eye can see, and the dots are close enough that your eye can’t tell them apart, so you just see the mixture.
Basically, if you have some red paint, and mix in blue paint, the pigment will become equally distributed so that some red and some blue get reflected, but they’re so close together that your brain interprets it as purple.
This is similar to how RGB Screens work, stand far away from your TV, and you can have a myriad of colours on the display, now put your nose up to the screen, suddenly you have just three colors
Simple explanation – by adding new paint into a mix you add new molecules that absorb portion of incoming light THUS subtracting portion of reflected light, giving new color.
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