why vehicle tyres are always black?

Although the natural colour of rubber is a milky white, the black colour we see was originally caused by soot. The soot was thought to increase the durability of the tyre along with cotton threads, which were inserted in order to reduce heat and increase stability. However, the main reason for black tyres on cars today, is the chemical compound ‘carbon black’. It is used as a stabilising chemical, which is combined with other polymers to create the tread compound of a tyre. 

The black color of tires comes from carbon black, a readily-available material that provides greatly improved wear characteristics and heat-dissipation capabilities when added to rubber compounds. Natural rubber is off-white in color, and in fact the first rubber tires were white.