CMYK colours:

The CMYK colours are the Process Colours of offset printing. Various image processing, drawing, and desktop publishing programs now have the capability of the colour separation of coloured images. The procedure of colour separation involves generating four black-and-white images (or negative images) corresponding to the four colours, cyan, magenta, yellow, and black. Then these separations are utilized photographically to generate the four plates for each of the four inks of the offset press. To produce the final colour image, each sheet is printed separately with each of the four-colour plates. As alignment is critical, accurate crosshairs are printed on each of the four-colour negatives to assist the printers in achieving good colour registry. 

We may express the conversion from an RGB representation to a CMY representation with the matrix transformation

where the white is represented in the RGB system as the unit column vector. Likewise, we convert from a CMY colour representation to an RGB representation along with the matrix transformation

where black is shown in the CMY system as the unit column vector.