I presume you're talking about LSR tires instead of raised white letters. I think a fast lacquer thinner on a rag will take the lettering off without harming the tire. I used it then to take the Goodyear off slicks. The fast thinner will evaporate instead of soaking in to the tire.
FWIW Years ago ( before Armor all ETC. ) when I had a service station, some of my customers would use brake fluid on their tires to make them black and shiny. They claimed brake fluid had preservatives to keep rubber soft and pliable.
