Could either be color fading or the yellow color pass wasn't complete for some reason.
Do you know about the 1988 Score promo cards?
https://www.comc.com/Cards/Baseball/...mos,sr,ot,i100
On COMC, the Larkin (green border instead of purple) sold for $50, Moreland $40, and Boyd $13.
PSA recognizes them as proof cards, and you can tell them apart because their stat lines for 1987 is all zeroes. Baseballcardpedia lists six cards in the set, but PSA has graded 2 Don Mattingly cards as proofs. Could be "mechanical errors."
http://www.psacard.com/pop/baseball-...e-proofs/93085