Nebraska
Nebraska lighting laws as they apply to neon are very simple:
1. No red or green lights visible from directly in front of the vehicle. 60-6229
** This is because some drivers might confuse the red or green as a signal from a traffic light, which could cause crashes
2. No color of light other than red ON the rear of the vehicle (except for directional signals, which may be red, yellow or amber). 60-6230 (2)
** Because if the signal lights were blue or purple, or any other color, other drivers might not realize it is a signal and may ignore it, which is hazardous
3. No flashing or rotating lights, except as specifically authorized for emergency vehicles, school buses, snow plows, etc. 60-6230 (1)
** So you cannot trick people or pretend to be an authority figure (like a cop), or speed
But that's all. So you can have any color under your car as long as it is NOT red or green, as long as they don't flash, and as long as you do not use them as directional signals.