Time. Time changes the chemicals in the plastic as they start to degrade and makes them change color. Other factors don't help, like sunlight and skin oils. But the less of those they come in contact with the longer it'll take for figures to change color, but it'll eventually happen. Could take years, or decades, but it'll happen. Such is time.
Not yellow, but mine & my best friend's Mattel Masterverse Ram-Man's pants are splitting/disintegrating at each ribbed section, by just standing in a display (meaning not from constant moving around). *sigh* It's what, barely a year old? 🙄
I'm not familiar with that figure but Mattel uses a kind of flexible, rubbery plastic for their WWE shirts and that's happened to a lot of my figures. I had a Hulk Hogan posed "ripping" his shirt open and after a month or so the plastic had hardened and was developing cracks. The old arm bands they had on the early Elite John Cenas were made of the same material also and they'd just dry up and crumble off after a while.
I guess these just aren't going to stand the test of time.
I have another result to report:
I forgot to take any "before" pictures, so there's no point is posting an "after" pic, but the pink on my DCUC Cosmic Boy's torso and upper legs had faded to a flesh tone that matched his face. It made him look like he was wearing the Grell Legion S&M outfit from that run.
I put him in the same solution and container as I did Lightray, and left him outside for a week because honestly, what did I have to lose.
I'm happy to report that surprisingly the pink has returned to its original color on those parts and he's back to looking like he did when I got him. So on some colors it appears it might work.
The Earth-2 Batman is next. Those same parts (always the torso and upper legs) are slightly yellowing and no longer gray. We'll see how that goes and report back.
Some of it has to do with the type of plastic they used. Like you noticed with DC the upper torso and upper legs. For me, when it was the Clone Troopers, it's normally just the body, not the appendages. I think in time, the rest of the figure parts would probably do it, but it'll take longer. It just happens to these parts faster because of the type of plastic.
I'm redoing my shelves and am coming across a lot of yellowed DCSH/DCUC figures. Not good. A lot of yellowed Batman figures. Like others have mentioned, torso and upper legs. Also, anyone notice really loose joints on figures that have been stored a long time? Dragged out some SM figures that have been stored for more than a decade and they are ragdolls now. Don't recall the joints being so loose initially. More plastic breakdown?
Pulled my Earth-2 Batman in yesterday, and as expected the grays have been restored to proper color. However, I don't know if I can recommend others doing it, as I noticed the blue around the neck faded a bit, as well as the black on the sides of his trunks. Slightly, but enough that I can tell. I had removed his cape, head, and arms so they're fine, but I get the feeling that the more blues that are in the chemicals, the more they'll fade. So definitely do that only at your own risk. Makes me hesitant about putting any Supermen in for an attempt.
Good thinking about removing at least the easy parts that don't need the treatment that you can. Does it help I wonder to keep the figure out of the mixture all together and just use the fumes.
That I don't know. But ultimately this whole experiment is a success because Lightray was my primary focus. The DCUC New Gods are the only ones from that line I have on permanent display. So getting him fixed was key.
I don't like that my DCUC collection has a time limit now. I gave up on McFarlane's DC line and sold the few I had off. I like Batman just fine but I want the entire DCU and mainly bronze age looks. Todd ain't doing right by me on those two wants. Supes is my favorite and we haven't even gotten a decent bronze age figure of him yet. Batman, Wonder Woman have though, if you can get them. I also don't like everything being a "Super Platinum Dilithium" store exclusive limited to 500 copies made! Guess I better get to painting if I plan on keeping the yellowed figures.
I haven't checked my dcuc stuff either. I probably should do that but I'm a little worried about what I may find.