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This has been covered before. The green color is due to the formation of copper chloride salts. The chloride comes from the PVC (poly vinyl chloride) either from unreacted monomer (vinyl chloride) or from the degradation of the PVC. The extent of degradation of PVC is due to a variety of factors...
1) nature of the plasticizer (the stuff that makes it soft and flexible
2) the amount of plasticizer
3) the type and amount of any other inhibitors (this may include colorants)
4) temperature
5) humidity
6) light
7) etc.
The formation of the copper chloride is a surface phenomenon that only goes down a few microns and doesn't affect the sound. In any product from any source that uses PVC what you've observed will occur. Alternatives would be to use an insulator made of polyethylene, polypropylene, teflon, kynar, etc. Note though that copper is a fairly reactive metal and the other insulators can have varying degrees of permeability to the atmosphere (gases can diffuse through the plastic) so that various atmospheric pollutants that may exist such as sulfides can also cause surface reactions.
Of course, if you use colored PVC or opaque varieties, it's a bit harder to see.
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