A person may not drive or move on any highway any vehicle or equipment that is equipped with or displays any light or signal device designed to emit an oscillating, rotating, blinking, or other type of emission of light, unless designated and authorized by the Administrator as indicated in paragraphs (1) through (9) of this subsection. The provisions of this section do not prohibit the display and use of any lighting device that may be permitted or required elsewhere in the Maryland Vehicle Law.
http://www.mayhem.com/~jacob/22218.html
Other states (like Maryland) phrase their law to read "Any unit designed to emit light", so lights are illegal whether they are on or off. Some states allow blue lights and prohibit red, and vice versa.
http://www.geocities.com/bruzilla/Laws.html
ILLEGALLY COLORED LIGHTS - Another violation which seems to be in abundance lately is vehicles with lights or lenses
which are in violation of the Maryland Vehicle Law. Officers are routinely stopping vehicles with Blue, Green and Purple
lights added to/or mounted in lieu of, the vehicle's original lights. (The small "laser" lights mounted to the hood are the most
common violation.) We're also finding many people who have removed the Red Lenses and Reflectors from the tail lights
of their cars, and installed CLEAR lenses instead. (Clear lenses are legal only if the bulbs themselves are still located
behind RED plastic, and the required minimum amount of RED Reflecting Material is installed in the clear lenses.)
The Maryland Vehicle Law clearly states the colors which are required on the front, back and sides of vehicles used on
public highways, and these laws are in conformance with the Federal Safety Standards for Motor Vehicles
manufactured in the United States. Changing or altering these required items subjects the vehicle operator to a
Moving Vehicle Traffic Citation, (usually $40-50) and/or a Safety Equipment Repair Order.
http://www.berwynheightspd.com/tinted windows.htm