I have seen a drop of liquid plumber turn copper green overnight, but this isn't a material I would want to work with - too hazardous for other than intended use. Some websites which discuss home made recipeis or have small volume commercial products (Patina-It) designed for hobbiests are listed below. I haven't tried any of these, so the risks and hazards are unknown. I think it would make a difference if you were trying to do a large area vs a small one in how the finish is likely to turn out.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E