The black you see is often a mix of grime from the fingers and oxidized metal. Green is typical of copper corrosion, which is why it shows up on bronze acoustic strings more than electric ones.
What speeds it up:
- Playing with sweaty or unwashed hands.
- High humidity or salty air.
- Leaving the guitar out without wiping the strings.
- Storing a guitar in a damp case.
How to slow it down:
- Wash and dry your hands before playing.
- Wipe the strings with a dry, clean cloth after every session.
- Keep the guitar in a stable environment.
- Use a coated set if corrosion is a constant problem.
Once strings are discolored and rough, replace them. Cleaning doesn't restore a corroded string. S.I.T. options: CS1046 in CRT Coated Nickel, CP1253 in CRT Coated Phosphor Bronze, and our GP-PLC Premium Microfiber Polish Cloth in Instrument Care for wiping down.