California Code, Vehicle Code - VEH § 25102

In addition to the lamps otherwise permitted by this chapter, any motor vehicle may be equipped with lamps on the sides thereof, visible from the side of the vehicle but not from the front or rear thereof, which lamps, together with mountings or receptacles, shall be set into depressions or recesses in the body of the vehicle and shall not protrude beyond or outside the body of the vehicle.  The light source in each of the lamps shall not exceed two candlepower and shall emit diffused light of any color, except that the color red is permitted only on authorized emergency vehicles.