I’m no expert when it comes to the printing process but here’s some evidence supporting it’s potential legitimacy.
Typically the printing industry uses cyan, yellow, magenta and black (like a household inkjet printer) to produce every colour. To get green, you’d mix yellow and cyan as seen below:
If you’ve ever seen one of those colour barcodes on printed product, they use that as quality control for checking ink levels. Here’s an example from an uncut sheet:
So my guess would be when they printed this card, they were low or out of yellow ink. Makes sense right??
Glad you asked! Check out the charmeleons from this post : www.elitefourum.com/t/error-charmeleon-not-sure-would-like-your-opinions/20902/1
Here’s the low-res version of what I’m talking about:
Going off what the Magic community thinks of this error (reference in that thread) it seems to be a legit error and would imply that the border is actually printed as a separate layer from the background of the card. So maybe the yellow ink was low when the background was printed and fine when the border was printed. The fact that the yellow border is present in full saturation and the backgound seems to be missing the yellow, I’m inclined to believe it is not a fading error.
ALTERNATIVE HYPOTHESIS:
Different inks fade faster than others. Maybe the yellow in the background faded first leaving the magenta and cyan. Perhaps the yellow border is printed in a different way and is more robust to fading. Then this could just be a result of sun fading. Can’t really say which idea is more accurate, I’m not an expert as I said.