Originally posted by Guu
Cyan, Magenta and Yellow if it's inks. Red/Yellow/Blue is nonsense really, and Red = Yellow + Magenta :p Printers will use black as well though, because the C/M/Y when mixed together give a muddy colour instead of black like it would with pure pigments. That's for subtractive colour mixing anyway.
I prefer Red/Green/Blue though, since that's what I'm more used to using and mixing when picking colours on the computer screen, as computer screens use the primary colours of light :p
In both those cases though, you can't make every colour in existance, which is why there's also other methods for describing colour space, like L*a*b*.