I am familiar with this problem. Are your rear tires a 60 series? It seems those are the ones that are browning. I had this problem on my Roadrunner, and a buddy had the same problem on his GTX. He contacted BFG, went for a tire inspection, and they exchanged his rear tires for him. Unfortunately, the replacements also turned brown. They again replaced them. Anyway, he got them exchanged for a third time and so far, so good. I called BFG about my tires, went down to my local Pep Boys for an inspection, after which they called BFG while I was there. I think they were going to replace mine for me, but unfortunately, I was not the original buyer, so they wouldn't help me out. My tires had only 900 miles on them however. There was something wrong in the production of these 60 series tires, which they supposedly have now corrected. You should check date of manufacture of any tire you buy, and make sure it was produced in 2016 to be on the safe side.
