CMYK are the four inks used in colour printing: cyan, magenta, yellow and key (black). Not only does the ‘K’ differentiate black from blue in RGB (red, green, blue), and avoid confusion with brown, it also describes the alignment of the cyan, magenta and yellow plates with the black key plate.
In CMYK, ink reduces light that would otherwise be reflected (subtractive), resulting in black where all colours are used in full (K is still required because the black achieved by mixing CMY is not as dense as K). In RGB, white is the result when all colours are used in full, whereas black is achieved when light is removed.
