The most common system for producing full colour print.
The vast majority of magazines and colour books are produced using four-colour process. Originally the artwork and originals were separated photographically using filters to produce four printing plates. Today the separation is carried out digitally.
The four ink colours are Cyan (Blue), Magenta (Red),Yellow and Black — often referred to as CMYK. Because the inks used are translucent (allowing light to pass through), they can be overprinted and combined in a variety of different proportions to produce a wide range of colours.
Theoretically it is possible to produce an adequate range of colours using just Cyan, Magenta and Yellow. Indeed for a time three Colour Process was a viable option. However, in practice much better results are achieved with the addition of black. The black plate is used to strengthen the shadow areas and reduce the amount of CMY inks required.
Although the range of colours which can be achieved is adequate for most jobs the process has its limitations. It is important to remember that many colours which are available as special inks have no close equivalent in four colour process. In some cases it may be necessary to print a fifth plate in order to match, for example, a particularly difficult company logo colour. The additional cost of this is normally prohibitive and the necessity should be avoided at the design stage.
It is not unusual, where an elaborate effect is required, to print in six or more colours. There are presses which are capable of printing eight different plates in a single run through the machine.