In describing the browning of my Diamondbacks, which I replaced with Cokers, I wasn't suggesting that the Cokers were entirely free of similar problems. Of the 8 Coker tires that I purchased at that time, 5 have stayed acceptably white, 2 have browned very noticeably, and 1 is quite brown -- although not as severely brown as the Diamondbacks. 4 of the acceptable tires are of a smaller size, with a narrower whitewall, than the others. None have more than a few hundred miles on them, and they are always garaged.
Having grown up taking care of whitewall tires, I'm frankly mystified by these problems. None of the tires I looked after during the first decades of my life ever browned or yellowed, despite plenty of hard use and harsh weather. The only grief I recall with them occurred as my dad frequently scuffed them against curbs -- which I often suspected he did intentionally.