Printing colour is consist of different percent of cyan,magenta,yellow and black,so you can also call it mixed-colour.Cyan,magenta,yellow and black is the original colour of printing.When printing original colours,it will mark own colour's net dot in its own plate.The net dot is produced by halftone screen and definitional original colours is consist of this four colour plates.We can make other colour by chage their size and distance in colour printing plate.Actually,this four colours are separate in the paper,but we can't distinguish this similar colours because of the limit of our eyes distinguish ability.The visual impression we get is mixed efficiency of each colour,so we can see different colours.
Yellow,magenta and cyan can almost compound all colours.However, it also need black colour when we printing ,because the compounded black is not pure,and it will be congeries if we compound black by yellow,magenta and cyan.It need pure black in printing.