Why are tires (or tyres) black?

Inspired by this thread and so many other “color” threads today… Why are tires black? I imagine it has something to do w/ heat, but I obviously don’t know. Don’t get me wrong, I’m glad they are black. I’m just wondering why they don’t come in a rainbow of flavors, er… colors?

Actually, I know you CAN get tires in different colors, at least for motorcycles. However, they won’t let us use them for racing. What’s up w/ that? Again, not that I want colored tires, just curious.

We just had this question two months ago.