In a question related to plugging in a device with a 6-20P Plug (240V Ground + 2 Hots) into a 10-30R (120/240V Neutral + 2 Hots) receptacle, I suggested replacing the outlet with a 6-20R and using the existing Neutral wire as a ground (moving it to the appropriate ground bus if necessary) by marking it as a ground on both ends.
I know it's ok to mark white wire with black tape or a marker when wiring switches, but I can't find any reference saying that it's ok to remark white or black (or any other color) as a grounding conductor.
Is it ever code compliant to mark small gauge (i.e. 10 to 14 AWG) insulated wire as a ground, or is it always required to be bare or green? I know there may be different rules for larger gauge wire, so this question is mainly about smaller gauge wire.