Rob, without going into the full chemical story again, when copper oxidizes(due to atmospheric oxygen)it turns brown and almost looks black as the reaction proceeds(e.g., very old pennies). A greenish color would be due to a reaction with other elements, most notably chlorine. The greenish color reported under some transparent speaker cables isn't due to atmospheric oxygen, which doesn't penetrate the jacket. Apparently some of the PVC(polyvinyl chloride)jackets(most are okay)were unstable and broke down so that chlorine reacted with the copper, forming the green chloride.
That's odd. I had a problem with 75 ohm sub wire recently (Esoteric Audio Luminis wire) and noticed that it had a green powder I would call corrosion for lack of a better term. After I pulled the tip off, it turns out the shielding braid (copper in appearance) is covered in green powder, but the central core of copper is still nice and red - although the wire is now 10 years old. But odd none the less as it has not bee exposed to chlorine. I only knew it had a problem because it sounded bad - i.e., would not always trigger the sub on, etc. I have some pics of the green powder coming, but upload is taking a long time.
