Bfg tires letters turn brown

I bought a set of 4 BFG's at the same time.

The two 215/70/14's have bright white raised letters that hardly ever need cleaning and never have gone brown.

The two 245/60/14's have letters that within two weeks go dark brown. It's really weird.

I've tried just about every product out there and have found that simple brake cleaner on a rag works best to get the brown off and is the cheapest method.