[citation][nom]loomis86[/nom]The primary colors are red, yellow, and blue. green is a mixture of blue and yellow.[/citation]
Red, green, and blue are the primary emissive colors (for things emitting light.)
Magenta, cyan, and yellow are the primary reflective colors (for paints and inks.)
Color perception is complex. The retina and primary visual cortex actually develops basically three channels, which correspond to a Red-Green axis, a Blue-Yellow axis, and a brightness axis. That is why you can't perceive Red or Green simultaneously (there is no such thing as "reddish-green"), or Blue or Yellow simultaneously (there is no such thing as "bluish-yellow).