There are two types of colors in the world: additive and subtractive.
Additive deals with the source: sun, stars, and light bulbs for example. These light sources mix red, green and blue wavelengths to produce white light. If you were to spotlights that slightly overlap one another of those three colors, where all three overlap would create white light.
Subtractive deals with objects that reflect light, so anything that isn't a light source itself. Consider your favorite shirt. Mine is red. When I wear my red shirt, or whatever color yours is, my shirt is actually absorbing every other color except for red. So technically, your shirt (or mine) is every color except what we perceive it as! If your shirt is black, that just means it absorbs every color.