Tire manufacturers blend into the tire polymer certain chemical ingredients which inhibit damage from ozone and ultraviolet light, the main environmental degradants of tires and all other types of synthetic and natural rubbers. Though more severe in cities and manufacturing centers, ozone is part of the air we breathe everywhere on earth. Hastened by the hazardous effects of UV light, ozone eventually causes rubber to dry and become brittle no matter the locale. The need to protect rubber against UV damage is why tires are black. Competitive absorbers have the added ability to convert harmful UV light wave energy into heat so it can dissipate harmlessly.
All tire manufacturers use the same competitive absorber, carbon black.
This is why tires are black and why tires are not available in designer colors. As carbon black loses the ability to do its job, it turns gray. This is why rubber grays as it ages. Ozone Tire manufacturers use waxes to protect against ozone. Ozone begins eating away the protective wax and before long reaches the tire polymer.