Black and or Green.
"Both colors are caused by oxidation as the copper reacts to elements in the environment. The green oxidation is commonly seen when pool chlorine is stored in the same room with electrical equipment and minimal ventilation, producing copper sulfide or hydrated copper sulfate. Jun 16, 2018."
Where you live and the environment is the biggest contributor to "GREEN". A product called NoCo is used on battery terminals, with great success for over 75 years. If you can minimize the O2 on the copper you can control the oxidation. The product is also conductive and semi-dries. It will last around acid battery terminals as long as you keep them treated. I've seen soft top batteries that were 50 years old.. No GREEN..
I'm with jasonbourne52, green is no good, I tossed a bunch of wire through the year because of "Green". I'm a mechanic, I see it all the time. I do a lot of rewiring because of aluminum wire with copper clad. The insulation is a problem too. Air tight is suppose to be air tight. That goop they use on connections works too.. No green in tractor trailer rigs when I'm done. I hate the ones that work on the coast and around salt...
Stereo cable, I use what ever the club is using. OCC weaves and ribbons seem to be the big thing now. They sure sound good. NO Green so far. I paid 39 or 59.00 dollars for the PCs