Underbody kits are legal......as long as the bulbs aren't visible. (At least in Ohio). No red/blue/green can be shown to the front of the car. No flashing lights (aside from turn signals/emergency flashers). Here, even the neon license plate frames are illegal because they alter the color of the license plate, but I've seen some of those that are red and only activate with the brake.......essentially another brakelight.
I don't know why they're putting them on anything but hondas though.
if you're interested in blue lights go to the forms @ mustangworld.com...theres a guy over there with a white stang, and he has ruined it by painting his whole inteior blue, blue lights all over, even blue freakin rims!!!! I CRIED!!!!