Actually... it depends on what you are talking about. If you consider light to be the perfect measure of color, then technically, white is all colors because white is what you see when an object reflects every color. Black is the absense of color because it is what you see when an object absorbs all light.
Thats why black shirts are always hotter in the summer than white shirts.
Now when you're talking about mixing paint, obviously if you mix every prime color you dont get white... but scientifically, white is every color of light mixed together.
---Andrew