I remember when BF Goodrich Radial TA tires had raised letters that were white like these Goodyears. Of course that was before Michelin purchased Goodrich and began producing raised brown letter tires.

255 60 or larger are brown. 235 60's are still white. Tires in photos on friend's RR have been mounted less than 2 weeks from new.
Brown letters on RR are 275 60's. White letters are fronts which are 235 60's.

All 4 new TA's on our 71 Vette are 3 months old on the car 255 60's and are dark brown.

This is a common problem with these tires. Believe I now have found the cause, but not yet certain.