I would avoid using any chemicals at all on G1 ponies like the plague. We know well by now that the plastic is not as stable in a 30+ year old toy as maybe it was years ago - it's impossible to predict the long term outcome if you add other chemicals to the mix. Quick fixes (like we learned with acne cream) can have long consequences. I personally wish people didn't use any form of bleach related chemical because we can't know the long term impact. It's a better rule of thumb to begin small and work up, not jump straight into the chemical level. Just a tip for the future.
Also, as the second image Galactica posted is my pony, I feel it's important to clarify that's not a proper comparison. My Twinkler is sun damaged on her display side but the pony pictured is a store pony. The OP is talking about MO Twinkler. MO Twinkler's plastic/glitter composition (as with all the MO Sparkles) is often more dense than that in the store pony release. That means all kinds of possible different ways in which she might discolour. Let's not forget that brown Nappers more commonly seem to occur in the green haired mail order version than the store version.