California Code, Vehicle Code - VEH § 25400

(a) Any vehicle may be equipped with a lamp or device on the exterior of the vehicle that emits a diffused nonglaring light of not more than 0.05 candela per square inch of area.

(b) Any diffused nonglaring light shall not display red to the front, but may display other colors. A diffused nonglaring light shall not resemble nor be installed within 12 inches or in such position as to interfere with the visibility or effectiveness of any required lamp, reflector, or other device upon the vehicle.

(c) A diffused nonglaring lamp or device, other than a display sign authorized by subdivision (d), shall be limited in size to an area of 720 square inches and where any lease, rental, or donation is involved the installation of the lamp or device shall be limited to those vehicles operated either primarily within business or residential districts or municipalities, or between business districts, residential districts, and municipalities in close proximity.

(d) An internally illuminated sign emitting not more than 0.25 candela per square inch and possessing copy which does not contain a white background may be displayed on each side, but not on the front or rear, of a trolley coach or of a bus being operated in urban or suburban service as described in Section 35107 of this code.