Cal. Code Regs. tit. 8, § 3321 – Identification of Piping.

(a) Where, in any one establishment, one or more hazardous substances are transported through pipe lines, all above ground piping systems used to transport gases, vapors, liquids, semi-liquids, or plastics shall be identified at points where confusion would introduce hazards to employees.
(b) Methods of Identification. Where identification is required for piping systems, one or more of the following methods shall be employed.
1. (1) Complete color painting of all visible parts of the pipe;
2. (2) Color bands, preferably 8 to 10 inches wide, at various intervals and at each outlet valve or connection.
  Where identification is provided by complete color painting or by color bands, a color code shall be posted at those locations where confusion would introduce hazards to employees.
3. (3) The names of or abbreviations of the names of the materials transported shall be lettered or stenciled on the pipe near the valves or outlets.
4. (4) Tags of metal or other suitable material naming the material transported shall be fastened securely to the system on or near the valve. Tag legibility shall be maintained.