Sorry to disappoint you, but color theory is in fact anything BUT simple. For starters, RGB are not "THE primaries in light." In fact, primary colors aren't a property of light but of the way you perceive light. You could just as successfully choose a different set of 3 primary colors and still be able to represent [an approximation of] any other color. That is, of course, assuming you're trying to represent color to a human with 3 types of cone cells - otherwise things only get more complicated...
