Red becomes the dominant color from Season 18 onward.
Season 18 was just before the movie in 2007, so that honestly syncs up perfectly with my theory that the actual start of Zombie Simpsons began close to that time instead of Season 9 or Season 11 like people often claim.
Really, I think the hate that Season 9-12 gets is all down to a handful of weak episodes (Kill the Alligator and Run, Hello Gutter Hello Fadder, Sunday Cruddy Sunday), people not fully getting the joke on the more meta episodes (The Principal and the Pauper, Saddlesore Galactica) or just sperging out over specific scenes in otherwise good or decent episodes (the panda rape in Homer vs. Dignity) plus the autistic ramblings of the Usenet spergs at SNPP at the time being taken as gospel truth.
Matter of fact, those autistic rants from the SNPP Usenet crowd are why Al Jean refuses to step down as the showrunner and gets adamantly angry at any suggestion of the show declining in quality. If anything, they just sped up the rise of Zombie Simpsons.
Anyone with a brain can tell you The Simpsons hasn't been good in at least a decade, and that's under the most charitable of definitions and that running a show for 30 years without severely jumping the shark is damn near impossible.
Yet in spite of all this, Al Jean is not only insistent about being showrunner until either he or the show dies, but he's still dead convinced that anyone who complains about the show's decline in quality in any way is just some Comic Book Guy-tier basement-dwelling neckbeard strawman.
Al Jean didn't cause Zombie Simpsons, at least not by himself, but he did exacerbate it in a lot of ways