To everyone who answered my question on how the primary colors of pigment are red, blue, and yellow; and on how the primary colors of light are red, blue, and green, thanks. But it doesn't really answer why they are what they are.
Erm.. I don't think I'm communicating myself well. Why don't green and purple combine to make blue, and why don't yellow light and cyan light merge to make green? Are these questions that can't really be answered?