If you shine red, blue and green lights on a wall, you get paler colours where they overlap. If you get the colour balance right, the overlap of all three colours will be white. This is called Additive colour.
If you mix paints, you get other colours. The basic colours for this are yellow, magenta (pinky purple) and cyan (pale greeny blue). As the colours mix, you get darker colours. This is called Subtractive colour. In real life, children usually end up with brown! It's easier with a computer.