Although the natural colour of rubber is a milky white, the black colour we see is because of carbon black.Actually Carbon Black is not added as a coloring agent,it is a filler.
Vulcanized rubber does not have very good wear resistance and hence a tire made of pure vulcanized rubber would wear out soon.
Carbon Black also increases the damping properties of rubber, thus transferring less shock loads to the automobile.
In addition to this Carbon Black is a good conductor of heat and would help conduct heat away from the surface with contact to the road and help maintain a healthy tire temperature.
An added bonus is that addition of carbon black increases the coefficient of friction with the road surface thus making it safer to drive.
Contributed By Pro Dinesh Nair, Calicut, ARPRO 2