CMYK is an abbreviation of cyan, magenta, yellow and key (black). These are the four process colours commonly used when printing.
We highly recommend using CMYK images as RGB images will be converted to CMYK in our workflow, and this may result in a small colour shift.
Colour settings are easy to change within the design package. For Adobe Photoshop, InDesign and Illustrator, you can change colour setting in the proof set up menu, under view.