I bought 4 new BFG's couple years ago. The rear tires were definitely different. the raised letters on all 4 would brown but only the fronts cleaned up white, NO MATTER WHAT I TRIED the rears were BROWN raised letters. Cleaning them made them change shade of brown to light brown while the fronts were bright white.

Discount tire replaced the rears this past May no charge.

I think what happens is the black rubber leaches into the white letters and they cannot be "cleaned".