I believe that the gold ink contains traces of copper which can oxidize and turn green when in the presence of other chemicals. A lot of these cards were probably exposed to various glues and pastes as well as water and who knows what other chemicals if they were ever removed from a scrapbook, etc.
I don't think that the borders without gold ink would be green. I assume they would be white as can be seen on a number of the minor league cards that have off centered ink registrations on the borders.
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