In Texas it'd be illegal. Same as tinted headlamps.
Depending on if you're in Texas or not, this should answer your question:
Any device that impairs the required effectiveness of headlamps, tail lamps, reflectors, etc is prohibited. The lights, both front and rear, are made by the manufacturer to meet this safety standard. An alternate or additional material, particularly something dark, would impair its effectiveness. Additional information on lighting can be found in the Texas Transportation Code, Chapter 547, Subchapters D & E.
The inspection criteria for tail lamps states: "Tail lamp. Inspect and reject if: lamp is obstructed by any part of the body."
Stop Lamps:
All motor vehicles are required to have at least two (2) stop lamps, unless vehicle was manufactured prior to 1960, in which case it is required to be equipped with one (1).
A stop lamp must emit a red or amber light, or any shade of color between red and amber.
A stop lamp must be visible from a distance of not less than 300 feet to the rear in normal sunlight.
The stop lamp shall be actuated upon application of the service (foot) brake and may be incorporated with one or more other rear lamps.
Stop lamp lens must be of a type meeting Department of Public Safety standards.
Tail Lamp:
Every motor vehicle shall be equipped with at least two tail lamps, unless vehicle was manufactured prior to 1960, in which case your vehicle is required to be equipped with one (1) tail lamp mounted on the rear which, when illuminated, shall emit a red light plainly visible from a distance of 1,000 feet to the rear.
