With light, it's RED, GREEN, and BLUE. That includes monitors, video projectors, and computers (gfx card hardware, I mean).
Back in primary/elementry school, you probably mixed red, yellow, and blue paint together (along with white and black and brown, to try and make the 'yuckiest' looking color possible)
With print (magazines, etc.) the colors are Cyan, Magenta, Yellow, and Black (K). Black is only required, because CMY looks crap (not dark enough) on its own.
I hope that helps a bit.
