RGB are the primary colors for additive (translucent) light, CMY are the primary colors for subtractive (reflective) light. The colored light you seen on your computer screen is additive; it's all made up of RGB. But printed ink is reflective, the light reflects off the page; it doesn't go THROUGH the page; since it is subtractive, it uses CMY. I suppose you could use Blue Red Yellow for printing; painters I believe use them as primaries; add b & w paint and you can make any color. Except for fluorescents and shiny gold, etc.