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.
As Alan pointed out, oxidation on the end of the wire can be easily removed with an abrasive such as sand paper, or the end can be trimmed and re-stripped.
The linked material by Mr. Russell is an excellent explanation of speaker wiring in general, in addition to the gauge table, and has undoubtedly helped many avoid wire rip-offs the past few years.
