I'm wondering if it might have to do with atmospheric ozone?
IF a belt starts "shifting", you'll know it well before failure, by a shaking and/or steering wheel vibration which is "little' and then gets worse. Depending upon how many miles you drive from the time you first notice it. Or the car will appear to "pull" to one side of the other, needing steering wheel "correction" to make the car drive straight.
It's not unusual for the black to bleed into the white on the white letters or whitewall areas, around the edges.
The tires which Coker has with the BFG Radial T/A-type tread pattern are wider whitewall tires, not the rwl BFGs.
That color change is not really a "quality of workmanship" warrantiable issue, to me. If the tire store does warranty it, knowing that when THEY turn it in for warranty to BFG, they'll get dinged for it.
Tires are composed of several different rubber compounds, depending upon which part of the tire it is. I suspect that's what you're seeing, graphically, with the "brown" area. Different rubber compounds.
I'd say clean them up and put some "tire black" or other type of coating on them, Make them look nice. As long as they don't lose air, drive reasonably smooth (with good balance), and don't have any cracks between the tread ribs, then you'll probably not have any problems with them.
DO verify the production date of the tires! This will be an indication of "tire age". Anything approaching 6 years old should probably be replaced, no matter how much tread is on them. You can use them as "roll-around" tires, though, but NOT for use on the public streets or highways.
CBODY67