CMYK is an abbreviation for Cyan, Magenta, Yellow, and BlacK. These are the ink colors used in 4-color printing, and differ from RGB colors, which are typically used for on-screen viewing. All documents destined for print use need to be submitted in CMYK, so that the printers can properly recognize all parts of your print-ready file. Leaving the colors as RGB can cause variance, drastically lowering the quality, accuracy, and consistency of the printed product.