Its the reason that most tires of any sort are black. Its a compound made mostly from tar. It makes for a stiffer, more resistand rubber. Even if you arent skidding the rear tire takes much more load than the front, so it will wear faster. The problem is compounded when you use a quick wearing tire on the back.

A lot of companies make a colored tire with a black strip down the center to get the best of both worlds. the fyxation brand ones are quite popular. They dont have cream, but do have white. http://www.fyxation.com/products