I tinted my tail lights with some spray on tint (not Nite Shades) and clear coat and it wore off within a year. I got pulled over three times for them- I carried around a list of the lighting laws in TX, which state pretty much the same as the CA one does; technically they are legal so long as they reflect red and shine red when braking- it is at the officers discretion to give a ticket or not (or to get pissed they were wrong about the laws and fuck with you for something else...). There are some cars that come from the factory with "dark" tail lights, those are DOT approved.
If i were to do it again, I would use Nite Shades and a better clear, and get some 50watt 1157 brake lights. Make 'em really shine!