[gizmo385]

Is it necessarily copper? There are other compounds that can produce green flames (like https://en.wikipedia.org/wiki/Triethylborane)

Edit: There are some other comments talking about "engine rich exhaust" that are burning parts of the engine. Still incredible they got it this far.

[marsokod]

The inside of the nozzle uses copper for heat dissipation. There were previous cases of green flames with the Raptor, and Elon said it was copper. The big question is why it was low on methane - bad calculation in the planned consumption, too methane rich during the ascent?