I know this is ground previously traveled. I know that different brands/models of tires behave differently. I did search and found some related questions and answers, but not exactly what I was looking for. Seems that Mike Phillips had something on SCG, but the link doesn't work and I don't know that there was anything more that he had there than here or on the megs.com FAQ's. Sorry this is long I tend to be verbose.
I'll further preface this by saying that I only work on my own, family, and friends cars and don't see a huge variety of tires. In my "research" on this topic in the past, I remember reading about the self-protective ingredients in tires that are released through sidewall flexing. As another bit of background, I sometimes will work on poorly cared for cars with filthy tires, which I usually use Eagle One All-Wheel and Tire Cleaner (what used to be A2Z), which is a harsh caustic (alkaline) cleaner. Usually I get this amazing brown muck that foams up when you scrub a(n already brown) tire like that, and am amazed at how dirty the tire was.
I have noticed on a friend's car with Michelin tires that the back tires got browner than the fronts, and I attributed this to the theory of the antiozonants etc. which migrate from the tire compound to the surface through flexing of the sidewall getting more action on the fronts than the back, thus the back tires were being affected and browning. I also have a car with Michelins, and recently (I've had the car for 3 years) noticed the same thing with a slight browning of the back tires. Since I had bought some Hi Temp Tire Cleaner from TOL to have a dedicated tire cleaner, I had tried it a couple of times just to fool around and wasn't sure if it had actually worked because the tires still seemed brown after.
Several weeks ago I decided to do a really thorough cleaning of the rear tires and make sure all the brown came off. They weren't really that bad, I maintain them pretty well. I mixed the Hi Temp at the 4:1 water/product ratio, which is the medium dilution. When I sprayed it on the tires and scrubbed with a tire brush, I got the same foamy brown gunk that I have seen with the A2Z, which surprised me a little, but I figured that "boy, this will get them clean". After the first go round, instead of looking cleaner, they looked more brown. I figured maybe old dressing was covering the brown surface, and I had just exposed more. I reapplied and rescrubbed...the brown foamy gunk seemed undiminished from the first time. I think I applied a third time, the brown foam seemed less, the tire seemed cleaner, so I went on to other washing chores. When the tires started to dry, they were still all brown, worse than when I started. The fact that I had so much brown foam and the tires weren't getting any better made me wonder if I was pulling the brown out of the tires. I made the command decision to keep scrubbing until they were clean, which took 5 or 6 applications total. At this point they seemed to be black, the foam wasn't brown anymore, however, I was so tired of scrubbing tires I didn't do anything to the fronts (other than my usual car wash scrub, I think).
I dressed all the tires identically, I have been using some Long Haul Tire Sealant that I got at Big Lots. I like this stuff pretty well (nice smell, decent look, big bottle for $3!), but I was starting to wonder if it doesn't have some silicone oil in it as well as the PDMS, since the dimethyl silicone oils seemed to be implicated in browning.
So a couple of weeks ago, I notice the back tires are starting to get traces of brown again (in the textured decorative part of the sidewall), while the fronts are good as always...and I'm thinking, what the heck, I scrubbed them for all that time...so I put some dressing on them...I don't remember if I used the Long Haul or Optimum or 303...so yesterday, the backs look terrible! I took some Pb's BnB and sprayed it on liberally and rubbed it in, and they look much better. This is bugging me, so I start searching again and reread and some posts (and the
Meg's FAQ) indicate the flexing action and "blooming" of the antiozonants is what turns the tire brown...so that blows my theory of why it's the backs and not the fronts.
Yesterday I was cleaning my friend Jr's summer wheels. In order to clean the season's accumulation of dressing (mostly to keep from getting black hands while polishing/sealing/handling), I used the Hi Temp Tire Cleaner...and got the same heavy brown foaming on both the inside and outside of the tires, even though the insides are only dressed when put away for the winter (with 303). After I was done, I swear they are brown tinged (those tires are not Michelins but Nokian WR's).
So what is the point of all this? I'm puzzled why I see more browning on the backs than the fronts (which I have seen on two cars, different Michelin models), why do the caustic tire cleaners seem to make this worse, are these caustics leaching the protective compounds out of the tire, and is that bad (I imagine there is only a limited amount of additive in the rubber)? What would be a better cleaner? P21S Total Auto Wash, or just using one of the neutral wheel cleaners like P21S or Griot's, or APC or Dawn (nooo!!! that will dry out your rubber....I'm kidding). Or is this from the dressings I'm using? I've been using the Long Haul recently. I don't remember it happening with the Optimum, but I do remember it when I used to use AA Detailer's Choice spray. I happen to take care of my mother's car, and even though I don't get to work on it very often, she seems to have no browning on her Goodyears that I use Zaino Z-16 on.
This is another of those topics (like paint issues) where I really wish there was someone from a tire mfg. on this board so we could get some answers straight from the horse's mouth.
I don't even bother dressing them much any more. IIRC I generally don't have browning issues with my snowtires either 
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