Hair roots contain pigment cells called melanin, which creates a black pigment. Melanophore is a chromatophore that sends pigment to new hair. The greater the amount of pigment sent to the hair, the darker the hair becomes. On the contrary, as the amount of pigment sent is reduced, the hair color turns brown and then blond. Some people believe that differences in hair color are caused by the differences in the intensity of ultraviolet rays contained in the sunlight, to which the hair is exposed.
Of course, now people can change the colour of their hair artificially, using dyes which lock themselves into the hair strands, giving a long term hair colour change.
Another artificial technique is perming, whereby curls are introduced into hair which may not necessarily have curls in it to start with.
This is done by reducing cysteine bonds in the hair polypeptide chains, so that the chains can move past each other in response to heat and pressure from a perming rod. The hair then undergoes atmospheric oxidation, where the cysteine bonds are reformed slowly after the chains have been moved about and a new structure is formed in the hair.
Finally, a lock-in chemical solution is used to seal the cuticles again, and holding in the polypeptide changes made to the hair strands, giving a permanent style to the hair.