California Code, Vehicle Code - VEH § 25281

A privately owned or operated water tender vehicle, when used exclusively for contract emergency services provided to any public agency, may display flashing amber warning lights to the front, sides, or rear of the vehicle when necessarily parked on a highway or other public road, blocking or partially blocking a highway or other public road, traveling at a speed slower than the normal flow of traffic, or crossing or entering a highway or other public road. The flashing amber lights shall not be displayed when the water tender vehicle is traveling to or from an emergency at the normal speed and flow of traffic, except when the vehicle is traveling in escort with a fire engine or other authorized emergency vehicle. The lights shall be covered with an opaque material when not being displayed.