Cal. Veh. Code § 25350

Any passenger common carrier motor vehicle manufactured prior to January 1, 1968, may be equipped with green identification lamps. Any bus may be equipped with an illuminated termini sign, an illuminated identification sign, or any combination thereof, which shall not project any glaring light. Internally illuminated termini signs, identification signs, or any combination thereof, meeting the requirements of Section 25400 may be mounted inside a bus. Any commercial vehicle, other than a passenger common carrier motor vehicle, may be equipped with an illuminated identification sign upon the front thereof which shall not exceed 24 inches in length or 8 inches in width and which emits diffused white light without glare.