Hi
I think the idea is that they didn't add any color at all - so what you get is just natural rubber. Which seems to be an odd pinkish/greyish/brownish sort of thing. I would imagine that it would rapidly pick up some color from dirt/grime and general wear.
Take a look at this photo taken by Hans Trauner of the Phalz in the Flugwerft Schleissheim, the Oberschleissheim branch of the Deutsches Museum. I was a little surprised at the pearly white appearence of this tire, but Hans assures me that they went to extreme lenghts to make sure that this A/C was as authentic as possible.
pretty interesting *

Mark