I had a set of BFG's on my 63 that no amount of scrubbing would get the dinge off the RWL's. I think they had some bad runs.
I also have recently had issues wit BFG white letter tires having a dingy appearance after cleaning them then driving the car heating the tires. I have read this is a normal process with BFG and to continue cleaning the tires and it will clear up. Although in the early BFG days I remember having the same issues with the white letters so we would use a 6” metal file to “dress “ our letters and they were extremely white. We used a mild less aggressive file with comet and a brush. This method still works for me today although I’m still experiencing the change in color after driving a while. My tires are new so I am hoping they will eventually stay white.