Why mixing paints doesn't produce the color white .

The accompanying figure represents the artist's color wheel.   Colors opposite one another are called complementary colors.   Those next to each other are adjacent colors.

A dab of paint is an opaque body.   Its color is due to reflection.   No paints are pure colors; they reflect light over a narrow wavelength band.   Yellow paint reflects yellow light, but also reflects small amounts of orange and green light, while absorbing red and blue light.   Red paint absorbs blue and yellow light, but reflects a bit of orange.   When yellow and red paints are mixed, the yellow paint absorbs the red light and the red paint absorbs the yellow light, but they both reflect orange.   So the mixture appears orange.   When two paints mix, the color produced is the color that both paints reflect.   Add some blue paint, which absorbs orange, to the red and yellow mixture and the paint loses its hue.   By mixing all your paints together, each pigment absorbs a particular wavelength and therefore no colors are reflected and the paint mixture appears a dark, dull gray.