As you probably know already, the underlying $10 puppet govt./occupation stamp doesn't exist in this color, as far as what few deeper references I have. There are unoverprinted proofs of that stamp but all I've ever seen was the sage green color, for whatever that is worth.
Rather than strict fading, best guess is that it's a color changeling caused by oxidation/sulfurization or other chemicals. I have no examples to show. If you care or dare to try, a short bath of a top corner in standard hydrogen peroxide might restore the green. That stuff will fade the violet postmark.