CMYK is the print colour mode. It is best not to print using colours set up using the RGB colour mode as these will be converted automatically and can provide unpredictable results. Most software used to create artwork provides the ability to set up colours using the CMYK colour mode.
In special cases, where you require exceptional colour quality/accuracy and so opt for a litho print process, “special” colours can be used. Specified using the Pantone Colour Matching System these colours are printed using inks mixed to specific colours. By its nature, this option tends to cost more than standard CMYK colour printing.