CMYK is a colour model that has the colours cyan, magenta, yellow and key which is black. Key is for black because B is all ready to blue so the K in black is for black. It is done with putting the four colours over each other to create the colours that are on the design. The order that they are printed in is cyan then magenta and then yellow with the black at the end. This is printed with halftone positives. This is done with this method because it is easer for the eye to make the sold colour form the dots and there can make seven colours when the three original colours are overlapping.
