2005 California Vehicle Code Sections 25400-25401 Article 10. Diffused Lights

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.
25401.  No diffused nonglaring light on a vehicle shall resemble any
official traffic control device.