APWA Utility Color Codes

The American Public Works Association (APWA) recommends the following guidelines be used when marking underground utilities.
White = Proposed Excavation
Pink = Temporary Survey Markings
Red = Electric Power Lines, Cables, Conduit and Lighting Cables
Yellow = Gas, Oil, Steam, Petroleum or Gaseous Materials
Orange = Communication, Alarm or Signal Lines, Cables or Conduit
Blue = Potable Water
Purple = Reclaimed Water, Irrigation and Slurry Lines
Green = Sewer and Drain Lines

Why are these colors used?
Often, you will see these colors used to identify utility lines in cities and on a university campus. Sometimes city or campus utility departments will paint the tops (such as manhole lids, or sewer drop-inlet tops) of their utilities these colors before an aerial survey is flown so that the people creating the maps can identify the type of utility. If you see utilities being painted in your area, there's a good chance an aerial survey is going to be flown soon. Or sometimes utility departments are just taking inventory of their assets and are painting to identify their utility infrastructure, or possibly to diagnose a problem and they need to identify multiple utility types in an given area.

The utility departments that mark their utilities usually use inverted spray paint, also known as "upside down paint". EngineerSupply sells inverted spray paint by the case.