What are the three primary colors of paint?

Red, yellow and blue is the traditional answer taught in schools, and it is the wrong one - printers use cyan, magenta and yellow, which genuinely mix to produce a far larger range. The red-yellow-blue scheme survives from eighteenth-century theory and from the fact that it is close enough for a paintbox.

The real complication is that there are two entirely different systems. Mixing light is additive: red, green and blue combine to make white, which is how a screen works. Mixing paint is subtractive: each pigment absorbs part of the spectrum and reflects the rest, so combining them removes more and more light and heads towards a muddy black. That is why mixing all the paints gives brown and mixing all the lights gives white.

There are also no true primaries in any strict sense. No set of three pigments can mix to reach every colour a human eye can see, and the gamut of any printer or screen is a compromise. Colour is a fact about the three types of cone in your retina rather than about light itself - which is why some animals with four or more receptor types see distinctions we have no words for and cannot reproduce.