What I find is that some are pretty discolored, either by time, inferior materials, or by (I guess?) vapors released from other surrounding toys.
First up, Action Master Shockwave losing his purple... and turning into what strangely looks like the Radio Shack version of his G1 original:


Up until 2 months ago, he was fine. But, after being packed away in a bin with other toys for one last move... this is his fate.
Notice too how it's primarily just from his front, the back apparently below the vapor level in the case.
Pretty frustrating stuff, ain't it?
How about you guys? Do you have pics of it happening to yours?