CMYK (Cyan, Magenta, Yellow, Key/Black) is the color model used in printing—colors are created by overprinting inks.

RGB (Red, Green, Blue) is the model used for screens—colors are created by mixing light.

A design created solely in RGB may look significantly different in print than it does on a monitor, because the color gamut is narrower in print. Professional print files are therefore always created in CMYK or converted to CMYK and delivered with embedded ICC color profiles.