I have done little research into the green borders because I always just thought they were a printing anomaly. The few I own are in my safety deposit box currently but know that the Tinker is a Sweet Cap Red 42 (listed on my inventory sheet b/c of HOF status). Give me a few days and I might be able to get over there and check the other backs.
Handling these cards as much as I do, I do not think it is a discoloration or "rusting" of the gold (real gold does not rust or tarnish although no one really knows what printed metal composite was used). I think it was actually printed with green borders.
And another card showing an incomplete gold application. On this one, you can actually see the white and yellow underneath. This is NOT my card.
Joshua