The rules for identifying grounded conductors (neutrals are usually grounded conductors, but they are not the only grounded conductors) are fairly straightforward. I have extracted the most pertinent sections of the NEC. There is more (like multi-conductor cables), but we would rarely run across them.
200.6 Means of Identifying Grounded Conductors.
(A) Sizes 6 AWG or Smaller. An insulated grounded conductor of 6 AWG or smaller shall be identified by a continuous white or gray outer finish or by three continuous white stripes on other than green insulation along its entire length.
(B) Sizes Larger Than 6 AWG. An insulated grounded conductor larger than 6 AWG shall be identified by one of the following means:
(1) By a continuous white or gray outer finish.
(2) By three continuous white stripes along its entire length on other than green insulation.
(3) At the time of installation, by a distinctive white or gray marking at its terminations. This marking shall encircle the conductor or insulation.
200.7 Use of Insulation of a White or Gray Color or
with Three Continuous White Stripes.
(A) General. The following shall be used only for the grounded circuit conductor, unless otherwise permitted in 200.7(B) and 200.7(C):
(1) A conductor with continuous white or gray covering
(2) A conductor with three continuous white stripes on other than green insulation
(3) A marking of white or gray color at the termination
The rules for equipment grounding conductors (EGC) are also relatively straightforward.
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.
(A) Conductors Larger Than 6 AWG. Equipment grounding conductors larger than 6 AWG shall comply with 250.119(A)(1) 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 is accessible.
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 exposed insulation or covering green
c. Marking the exposed insulation or covering with green tape or green adhesive labels
To summarize the NEC rules.
You cannot use a white or gray conductor for anything other than a grounded conductor (i.e.- white or gray conductors cannot be re-identified).
In sizes #6 or smaller, a grounded conductor has to be white or gray.
In sizes larger than #6 you can re-identify a conductor as a grounded conductor, other than a green one, typically with a band of white tape at both ends.
You cannot use green or green with yellow stripes for anything other than an EGC (i.e.-they cannot be re-identified).
For conductors larger then #6 you can identify an EGC that is not green by painting or covering the exposed insulation green, removing the exposed insulation, or marking the insulation with green tape or labels. Note however that the prohibition on using white or gray for anything other than a grounded conductor still applies so you cannot re-identify a white or gray conductor for use as an EGC.
Note that the NEC does not specify any other color coding requirements for wiring other than the use of orange for high legs on mid-point grounded delta systems.
