RGB is not the only color model you will come across. In four color process printing (used by commercial printers and many output devices) the secondary (subtractive colors) colored inks are combined to represent color.
Because all printing inks contain some impurities, these three inks actually produce a muddy brown and must be combined with black (K) ink to produce a true black. (The letter K is used to avoid confusion, because B might also stand for blue.)
If you intend to send an RGB image to a litho printer then you should convert it in your page layout program first. Remember, once converted to CMYK you cannot regain the original RGB color.