CMYK is the standard for digital printing. The letters literally stand for the inks that are used in the printers- Cyan, Magenta, Yellow and Black. These inks, when blended, create an unlimited variation of colors but are less reliable than using spot colors- or the PMS color system. Spot or PMS colors are very exact. Because of the exact nature of spot colors, using spot or PMS colors can be a more expensive print process - especially if you are printing many spot colors at a time. When you have art with more than 4 spot colors- or complicated art, it is more cost effective to use CMYK digital printing. If you need help navigating this- don't worry! We are experts in color and print!
RGB is the color standard reserved for screens. Red, Green, Blue. Using RGB colors is not preferred for printing and can produce unexpected results. Remember, when viewing art proofs on your screen that each screen is manufactured and calibrated differently- so a print proof can look different on your computer screen or phone than it will look when actually printed in ink.