Seems to me it's related to heat in the tires: the lettering on my slicks would be browner nearer the "tread" than it was near the rims.
I'd wipe off the "Goodyear" lettering with lacquer thinner - hey, I'd advertise for them if they were free...

In any case, I'm not much of a white-letter guy anyway; I think cars look visually cleaner/less "busy", so to speak, without lettered tires.
But for something like a '70 F60 restored car, you're kind of stuck with the letters.