When they turn brown just break out the lacquer thinner and a rag and wipe them off. The BFG's with all their shortcomings is still the best looking tire for our cars IMHO.
Lacquer thinner won't work. The brown is from the rubber leaching through the letters. The brown is not on the surface, it is all the way through. Nothing works to make them white again once they go brown except a paint pen.
I have BFG, GoodYear, and repro Polyglas RWLT's on my toys that all brown......no paint pen needed, Trichloroethylene, a rag and one's finger, whitens the crap out of the letters, but you need to be careful to do only the letters...the plus side is it last about 2-3 yrs before they brown again....the "browning" is nothing new, I recall doing Wide whites and Standard white walls and RWLT's back in the 60-80's on up to the present to remove "browning"...
As for the OP's request, I'd avoid all the crap RWLT's on the market today, invest in a quality black wall performance tire and contact one of the many white lettered applique type letter/number makers and apply their offerings to the surface of a quality tire....some vendors also custom make letter/number appliques to your specs
