Interesting you posted this because I recently looked at some of my celestias under a black light for fun, I noticed one had a white body and pink head! Another had a white head and a pink body! xD If that helps? Maybe different types of plastic effect how they sunfade. Have you tried benzoyl peroxide? I heard that helps just a little with color bleaching!
Sorry your celestia didnt fade correctly....but hey that lavender ombre on her legs looks pretty awesome!!!!