This help?
200.6 Means of Identifying Grounded Conductors.
(A) Sizes 6 AWG or Smaller. An insulated grounded con-
ductor of 6 AWG or smaller shall be identified by a con-
tinuous 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.
(I don't think it matters for the ungrounded conductors, but local rules, etc... this ref was easy to find.)