By law, you can't modify or cover any portion of the taillight/sidemarket/headlight REFLECTORS (not the whole light itself). However, like speedminded said, there is a blanket coverall for modifying dot approved parts anyway.

Realistically, you can probably get away with it as long as light shines through clearly. It's a safety thing. I haven't yet got ticketed for these, but then again, light goes right through em, and I've got a plain set that isnt painted just for fix it tickets (though I haven't gotten one of those either) Ultimately, its up to the police officer to ticket you or not. Price you pay to play the game, its not really a big deal.