California vehicle code.
"26708.5. (a) No person shall place, install, affix, or apply any
transparent material upon the windshield, or side or rear windows, of
any motor vehicle if the material alters the color or reduces the
light transmittance of the windshield or side or rear windows, except
as provided in subdivision (b) or (c) of Section 26708.
(b) Tinted safety glass may be installed in a vehicle if (1) the
glass complies with motor vehicle safety standards of the United
States Department of Transportation for safety glazing materials, and
(2) the glass is installed in a location permitted by those
standards for the particular type of glass used."
http://www.dmv.ca.gov/pubs/vctop/d12/vc26708_5.htm
"Sun Screening Devices: Requirements
26708.2. Sun screening devices permitted by paragraph #10 of subdivision (b) of Section 26708 shall meet the following requirements:
(a) The devices shall be held in place by means allowing ready removal from the window area, such as a frame, a rigid material with temporary fasteners, or a flexible roller shade.
(b) Devices utilizing transparent material shall be green, gray, or a neutral smoke in color and shall have a luminous transmittance of not less than 35 percent.
(c) Devices utilizing nontransparent louvers or other alternating patterns of opaque and open sections shall have an essentially uniform pattern over the entire surface, except for framing and supports. At least 35 percent of the device area shall be open and no individual louver or opaque section shall have a projected vertical dimension exceeding 3/16 inch.
(d) The devices shall not have a reflective quality exceeding 35 percent on either the inner or outer surface. "
http://www.dmv.ca.gov/pubs/vctop/d12/vc26708_2.htm