I still remember the exhaust plume of one of the experimental Starships turning green. "Whoa, it is eating itself!" And verily it was.

yeah copper burns green and is commonly used in liners and other components. If it starts to burn, which it's not suppose to, you see green. You'll see a green flash when the merlin engines startup but that's because of the hypergolic fluids used to get the pumps running and ignition started, not copper burning. During Falcon night launches you can really see the green glow of the starter fluid on ignition.