RGB does stand for Red,Green Blue but they are not regarded as "print" colours when used in Photoshop.In photoshop RGB and CMYK are used to define two different colour systems, Additive & Subtractive.Additive is a way we see colour with direct light,such as the colour on a tv screen .Its called additive because when you add all the colours in light together to get white.Subtractive is the colour system used not only in printing,but also in traditional painting where the more colours(pigments) you have the closer you get to black, hence the name as you subtract colours to get white. Printing uses the subtractive system and uses the four coloured inks Cyan,Magenta,Yellow and Black..(k for black comes from the old printing term of Key or Keyline to describe the black plate.) Moniters and TV,s use the Additive system ,combining three coloured lights red,green and blue.Because of factors such as luminosity not all colours can translate from RGB to CMYK.This is why those amazing light effects on the computer screen just look like washed out paper when printed. So if your work is intended for onscreen viewing just stick to RGB,but if you intend to print it you should work either in CMYK or at least with the CMYK preview on to avoid disapointment.