This is something someone at a tire factory may actually know more specifically, but I had always heard natural rubber used in tires is really white (we probably all know that), and to get them black the tire makers used, or possibly still use, charcoal as a black pigment color additive. We've all have seen some RWL tires that get brown or browner than others. My tire guy years ago, told me if I want to keep my white letter tires white (winter storage tires/wheels) to take a piece of large cardboard and place between each tire before I stack them. The white rubber absorbs the black pigment from the other black tires when sitting for long periods. This is why the blue stuff we have washed off our white-walls and white letter tires is there. Likewise, the white letters will start to absorb the black around the letter too. I had one tire out of 4 that was browner than the others - I just couldn't get it white. As a last resort, I took a metal file and filed down each white letter to bring them back to white again. It worked. Yes, I also use Westley's and a hand brush each year on my BFGs then spray with a top-coat of tire shine. The key to good tire shine pre-show is to spray it on the night before, heavy, and then DON'T touch it. Leave it alone and then when you come out the next day, the tire shine is either absorbed or evaporated leaving a nice semi-flat black and your wheels and tires are gorgeous...and it pretty much stays that way the rest of the year depending on how much you drive your car.