RGB refers to the additive color space, Red, Green and Blue, this color space is used primarily in monitors, television screens, digital cameras, scanners etc. When all three of these colors are combined they produce white. CMYK refers to subtractive color space used in process printing: Cyan, Magenta, Yellow, and Black. These are the inks used in the 4-color process printing commonly referred to as full color printing. When these four colors are combined they produce black. Because of these fundamental differences in the two processes it is impossible to exactly match colors from one color space to the next.