I call it a ground conductor as does the NEC. It took me a while to get used to calling it a ground cond. I consider it a redundant ground. I do not know when it went into the NEC, or the reasoning. I would like to see the actual proposal that was accepted by the NEC panel at the time. It certainly would help me understand the logic behind the rule, and to give a factual response to someone asking why I enforce that rule, apparently a lot of AHJ do NOT enforce this. How is this enforced in other areas? I have wondered if the rule is there possibly because of the potential for the ground cond in the cable to deteriorate and fail at the connection to the lamp. As for insulation color, most use black or green. I am not fussy about that. Just getting electricians to install a redundant ground and treating the connections with compound is a big accomplishment.
Harold, you are right, and I am aware of the luminaires rated for above 18" in above ground pools. Do you know of any wet niche lights that are rated for use above 18"? I have had pool people tell me (after they have been cited) they will replace the nonconforming light with one for use above 18", I tell them I want the paperwork that goes with the light. To this day I have not seen any light (wet niche) rated for above 18", or the paperwork.