The CMYK colour model
The CMYK colour model (process colour, four colour) is a subtractive colour model, used in colour printing, and refers to the four inks used in colour printing: cyan, magenta, yellow, and key black. Though it varies by print house, press operator, press manufacturer and press run, ink is typically applied in the order of the abbreviation.
The "K" in CMYK stands for key since in four-color printing cyan, magenta, and yellow printing plates are carefully keyed or aligned with the key of the black key plate.
In additive colour models such as RGB, white is the "additive" combination of all primary coloured lights, while black is the absence of light. In the CMYK model, it is the opposite: white is the natural colour of the paper or other background, while black results from a full combination of coloured inks.