CMYK

Q. Do I need to use CMYK color mode?
A. Yes. If your files are submitted using any other color standard, such as RGB or Pantone, they will be converted to CMYK during preflighting. Conversion from one color standard to another may cause colors to shift. Before submitting your digital artwork files.
RGB is the digital colour gamma wich is Red, Green and Blue.
CMYK is the four main printing process colours and they are Cyan, Magenta, Yellow and Black.