At the end of the day it's about safety. So it pays to account for the user's force-of-will. If a code violation of one type or another is inevitable, it is a matter of least harm.
Let's first look at 250.119.
250.119 Identification of Equipment Grounding Conductors. Unless required elsewhere in this Code, equipment grounding conductors shall be permitted to be bare, covered, or insulated. Individually covered or insulated equipment grounding conductors shall have a continuous outer finish that is either green or green with one or more yellow stripes except as permitted in this section. Conductors with insulation or individual covering that is green, green with one or more yellow stripes, or otherwise identified as permitted by this section shall not be used for ungrounded or grounded circuit conductors.
My first read of "Continuous outer covering" was "more than the usual band of black tape you find on switch loops", e.g. covering the wire with many spiraling loops of electrical tape, paint, or shrink tubing, to the full length it is visible in a box or legal splicing point. With the notion that the mark survive the kind of manhandling that often removes that black band. That may not be what code means, but now, back to our end-user who is going to solve his problem by some code violation or another.
The rationale of code allowing exception for very heavy wire seems to be that very heavy wire is expensive. Well, if General Motors can't afford to replace a 0000ga. wire simply because it is not green, maybe Joe Poor can't afford to replace a 12ga.
Let's see how that rationale holds up against Code.
250.119(A) Conductors Larger Than 6 AWG. Equipment grounding conductors larger than 6 AWG shall comply with 250.l19(A)(l) and (A)(2).
(1) An insulated or covered conductor larger than 6 AWG shall be permitted, at the time of installation, to be permanently identified as an equipment grounding conductor at each end and at every point where the conductor
Exception: Conductors larger than 6 AWG shall not be required to be marked in conduit bodies that contain no splices or unused hubs.
(2) Identification shall encircle the conductor and shall be accomplished by one of the following:
- a. Stripping the insulation or covering from the entire exposed length
- b. Coloring the insulation or covering green at the termination
- c. Marking the insulation or covering with green tape or green adhesive labels at the termination
Imagine the three occurrences of "larger than 6 AWG" weren't there. If so, this would authorize this kind of marking. And it doesn't specifically say "... and this is not allowed for <=6 AWG." However, if the Code meant that, those three occurrences would be redundant. The fact that it is there implies the prohibition.