[orost]

Copper burns green, and the engine contains a lot of copper, so I think we saw a malfunctioning Raptor burning itself up.

Other answers mention TEB starter fluid but I don't think that's right, because as far as I know Raptor has spark ignition only.

edit: Elon Musk just tweeted that the immediate cause of the crash was low pressure in the fuel tank, so that would make sense, as running oxygen-rich would overheat and melt the engines.