I don't know for sure, but I have a MIP mail order that looks like it has a couple of small white spots. I could see her slipping thru the cracks, so to speak, as a mail order more than any other release, too.
None of mine have turned more white over the years I've had them, which I assume makes it more likely the dye was wrong from the start, rather than fading?
A white headed blue belle owned by clarisesponies ended at $61 last week on eBay, I'd expect yours would go about the same?
*blushes* I may be slightly obsessed with these oddballs, haha!