Is Underglow Neon Legal in the US?

Why?

Unlike emissions, underglow is governed by state law, so the answer genuinely depends on where you drive. A number of states allow it in restricted form, some allow it only while parked, and a few effectively prohibit it on the road.

Where it is allowed, the recurring restrictions are about color and motion: red and blue are almost always banned because they mimic emergency vehicles, flashing or rotating lights are prohibited, and the setup must not distract other drivers or obscure required lamps. Steady white or amber is the safest choice.

Because the rules and the list of permitted colors change at the state line, the same underglow kit can be fine in one state and illegal in the next. Check your state's specific lighting statute before running it on the road.