Whether you’re painting your house, or colouring your customer’s hair: colour is colour. Red is red, yellow is yellow and blue is blue. By mixing these primary colours, you can produce a rainbow of secondary colours and subsequently an unlimited palette of variations on that. Good knowledge of colours will allow you to provide the colours with greater depth and reduce unwanted colour effects.
Note: always take the natural hair colour of your customer into account when choosing a hair colour. After all, the end result will be determined by a combination of both these factors!