text: Degree in the arts, though I work in software.  
  
  The short story is that RGB is light (transmissive), think your TV or a projector, or slide film, if you are old enough. CMY is ink sitting on some sort of a substrate, so reflective.  Due to the inks not being pure, most commercial printing applications would actually use CMYK(blacK). Without the Black ink, 100% CMY would look kind of a muddy brown, so the Black ink is used to make it look like black, well, that and that much ink would be very very thick.