Well one of the reasons is you are seeing wide spectrum light selectively reflected back to you by the paint as colors whereas colored lights are set to emit those colors in the first place before they mix with anything.

Additive vs Subtractive color mixing

Additive is emitted wavelengths, Subtractive is absorbed wavelenghts.

spectralore.com/…/additive-vs-subtractive-color/

The key that got me to understand is that our eyes have only the three types of photoreceptor cells that detect color (a.k.a. cones): Red, green, and blue. We don’t have cones that see yellow, for instance.

Red light has wavelengths in the 625-750 nanometer range, and green is 500-565nm. Yellow is between red and green at 565-590nm. The ranges that our cones detect are kind of fuzzy, and imprecise. Which means that yellow light will stimulate the red codes a little bit, and stimulate the green cones a little bit, too. Then, our brains combine the signal from the red and green cones to extrapolate that the light is actually yellow.

But that means our eyes can be easily fooled. If we mix a red light that stimulates our red cones with a green light that stimulates our green cones, our brains receive the same neural signals as if it were yellow light, and so we perceive yellow. The same holds true for all other non-RGB colors.

Mixing paints is an entirely different phenomenon, though. Red paint absorbs all colors of light except red, so the light that bounces off and reaches our eyes consists of the red wavelengths. Green paint absorbs all colors of light except green. When you mix red and green paint together, the green paint absorbs the red light, and the red paint absorbs the green light, and both of them absorb all of the other wavelengths. They absorb light imperfectly, so the result that we see is some sort of muddled brown. Mix enough paint colors together, and the resulting paint absorbs almost all wavelengths of light, so it looks black.

I guess you could think about it as where the mixing happens, whether the mixing happens out in the world, and the resulting light reaches our eyes, or whether the light reaches our eyes first, and the mixing happens there.

There was a thorough explanation in a different NSQ community just a few weeks ago

GCompris has some games to help you understand.

lights emit the color of light they are. substances absorb the colors they aren’t. Adding all colored lights together ends up with a white light source while blending all colors theoretically leads to black (although in my experience its hard to not end up with gray or brown. to be fair I don’t really have experience blending light into white light.)