Use a fine grit ScotchBrite pad in a drill motor or a die grinder. Grind the yellow layer away to expose the white layer underneath. That's how the white rubber is exposed in the first place- - - -the white is covered by black rubber during the curing process. When I worked at Armstrong Rubber Co. in the early 1980's we produced 14K tires a day, and every whitewall had the white layer exposed by a grinding wheel.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!