Most of the people that I have worked with over the years have attributed the green color observed in some fiberglass to the "finish" that was applied to the glass. The finish applied to a glass depends upon projected resin use, epoxy, polyester, silicon, phenolic and compatibility/durability of the bond . The green color is normally associated with the "Volan A (contains chromium complex)" finish. The Volan A finish has been used for years (decades) and is compatible with epoxy and polyester resins. There are "newer" silane finishes but there are still specifications that require Volan A.