What do the color designations of wire mean?
Ungrounded "hot" conductors can be identified by any means that is not used to identify grounded "neutral", or grounding conductors. So basically if it's not white, gray, any color other than green with three continuous white or gray stripes, green, green with yellow stripes, bare, or any of the other identification methods below, it can be an ungrounded "hot" conductor.

Blue is an acceptable color for the insulation on an ungrounded "hot" conductor.


  National Electrical Code 2014
  
  Chapter 2 Wiring and Protection
  
  Article 200 Use and Identification of Grounded Conductors
  
  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 one of the following means:
  
  (1) A continuous white outer finish.
  
  (2) A continuous gray outer finish.
  
  (3) Three continuous white or gray stripes along the conductor's entire length on other than green insulation.
  
  (4) Wires that have their outer covering finished to show a white or gray color but have colored tracer threads in the braid identifying the source of manufacture shall be considered as meeting the provisions of this section.
  
  (5) The grounded conductor of a mineral-insulated, metal-sheathed
  cable shall be identified at the time of installation
  by distinctive marking at its terminations.
  
  (6) A single-conductor, sunlight-resistant, outdoor-rated
  cable used as a grounded conductor in photovoltaic
  power systems as permitted by 690.31 shall be identified
  at the time of installation by distinctive white
  marking at all terminations.
  
  (7) Fixture wire shall comply with the requirements for
  grounded conductor identification as specified in 402.8.
  
  (8) For aerial cable, the identification shall be as above, or
  by means of a ridge located on the exterior of the cable
  so as to identify it.
  
  Article 250 Grounding and Bonding
  
  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.