Can duct tape be used to mark neutral wires?
We can always go for the old chestnuts.  They're on page 1 and 2. 


  110.2 Approval. The conductors and equipment required or permitted by this Code shall be acceptable only if approved.
  
  110.3 Examination, Identification, Installation, and Use
  of Equipment.  
  
  (A) Examination. In judging equipment, considerations
  such as the following shall be evaluated:
  (l) Suitability for installation and use in conformity with
  the provisions of this Code
  (4) Electrical insulation
  (5) Heating effects under normal conditions of use and also
  under abnormal conditions likely to arise in service 
  
  (B) Installation and Use. Listed or labeled equipment
  shall be installed and used in accordance with any instructions
  included in the listing or labeling. 
  
  110.12 Mechanical Execution of Work. Electrical equipment
  shall be installed in a neat and workmanlike manner.


There's also a problem with the gray/silver color. 


  200.7(C) Circuits of 50 volts or more.  The use of insulation
  that is white or gray or that has three continuous white
  stripes for other than a neutral for circuits of
  50 volts or more shall be permitted only as in (l) and (2).
  (1) If part of a cable assembly that has the insulation permanently
  reidentified to indicate its use as a hot by marking tape, painting, or other
  effective means at its termination and at each location
  where the conductor is visible and accessible. Identification
  shall encircle the insulation and shall be a color
  other than white, gray, or green.
  (2) [pertains to cordage only]