There are three primary colours in pigments - Red, Yellow and Blue. There are two ways to produce more colours:  

Physical Mixing: Physically mix colours to produce a new colour. This is difficult for printers since their colours are quick drying and so colours to be mixed should be applied concurrently.  

Optical Mixing: Mixing to give illusion of a new colour. This can be done in various ways:  

• Apply colours one upon another. This is done employing inks that are somewhat transparent as modern inks are.
• Applying dots of different colours so close to one another that human eye can't discriminate the difference. This is theory behind Dithering.