My [limited] understanding is that any card made 20-30 years ago uses paper that has titanium dioxide in it, which will fluoresce very white under UV light. That's one way of outing counterfeit Fro-Joy cards. Perhaps the focus of inquiry should be the paper [front and back] and not the ink.
Assuming the card is legit, however, my view is that it would be best classified as a printing freak or variation, not as a legitimate new back type. I'd treat the other accidental back color variations as such too. Doesn't mean it isn't desirable...
Last edited by Exhibitman; 08-22-2012 at 03:50 PM.
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