Colour images need to be separated into primary colours in order to be printed. The primary ink colours create the four colour process, consisting of Cyan, Magenta, Yellow and Black.

Printing presses can only print solid colour in the image area, while no ink prints in the non-image areas. To achieve the required result, the image is converted into a pattern of very small and clearly defined dots. This process is called screening. The printed result is an optical illusion, relying on the eye to mix the dots of the four process colours.

Special colours are those which are independent from the four colour process. Some examples are metallic, fluorescent and Pantone (PMS) inks, commonly known as spot colours.

The four colour process cannot always adequately simulate special colours. At times, results can be quite similar, but in many cases it is not accurate enough, and additional special colour inks are used to achieve the required result.