Graphics generally use RGB (Red, Green, Blue) or CMYK colors. Graphics on the Web or designed for on-screen display are RGB images. Because offset printing utilitizes CMYK inks, all full-color graphics need to be saved with CMYK colors. This simple conversion is done in your graphics software program.
CMYK
To reproduce full-color photographic images, typical printing presses use 4 colors of ink. The four inks are placed on the paper in layers of dots that combine to create the illusion of many more colors. CMYK refers to the 4 ink colors used by the printing press. C is cyan (blue), M is magenta (red), Y is yellow, and K is black, the key plate or keyline color.
A mistake often made when submitting artwork for 4-color printing is not converting the images to the CMYK color space. This is needed so that the file can be separated into the four colors (see example) so that a separate printing plate can be made for each of the colors.