Will a regular urethane clear be optimal for nightshades on the taillights or is there another kind that I could use that would be better? Also, what type of prep work should be used on the tails before/after applying? I want to make sure they come out great, not just ok. I searched and the only thing I could find is "make sure to spray thin coats".

I'm probably not gonna be much help, but I used Testors High Gloss Clear. I had tried another one called something like Krylon Triple Thick Clear Glaze and it reacted badly with the the Nightshades and got very dull and rough. I didn't do any prep other than making sure the lights were free of dust, fingerprints, etc, shot about 4 light coats and I'd call my results "pretty decent", probably not as professional as you're looking to accomplish. Some sanding in between coats probably would have done a some good, but I didn't trust myself not to screw it up and my car is far from a show car anyway.

Thank you for your reply. I have the means to use a spray gun and clear them that way for no cost, so I was probably going to go that route. I'm not going to go as dark as yours are, or at least how they look in that pic. Just darker than normal. I'm painting my car red and I just think stock tails and this particular red will look funny. Plus I'm looking to get rid of these cf euro's.