I have looked at lots of GECs, they called me the grounding inspector. Green tape over black THHN is the norm if it was insulated at all. That pretty much means it could be any color with green tape. I even think it could be white by the definition of "grounded conductor"
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Grounded Conductor. A system or circuit conductor that is intentionally grounded.
but the confusion factor would have me asking for a penny's worth of green tape. [Linked Image]

Greg Fretwell