My hair is black and had never attempted to put some colors on it. But many people (especially women) do this to change their personality. Being blonds may entail having more fun or brunettes looks more sophisticated.
Or is it the other way around. Woman who had their hair dye simply because they want their personality to be reflected on their hair color. Therefore, they can change their hair color to match their personality, rather than trying to match their personality to their hair.