Usage of neon underglow lighting is governed by Washington Administrative Code, Title 204: State Patrol (Equipment), Chapter 204-21: Lighting Requirements.
Is neon underglow legal in Washington?
Washington law restricts additional aftermarket vehicle lighting which includes neon underglow. In Washington neon underglow is not legal on public roads.
Using car underglow is legal only when parked on a private property. Even on private propery you must ensure your lights are not displaying red or blue colors, nor flashing or rotating in any way.
Washington vehicle lighting laws
Below are all relevant excerpts from Washington Vehicle Code that limit, restrict or allow certain aftermarket lights to be installed on vehicles.
WAC 204-21-230 – Lighting equipment prohibited.
[…] (a) If a vehicle is in motion on a public roadway, the vehicle must not:
(i) Display aftermarket neon lighting devices.
[…] (b) If a vehicle is not in motion and parked on private property, the vehicle may use aftermarket lighting except as outlined under RCW 46.37.180. […]
(3) Red emergency lights are prohibited on any vehicle other than an authorized emergency vehicle, a law enforcement vehicle, an emergency tow truck as defined in WAC 204-21-020(8), school buses, and private carrier buses.
(4) Blue lights are prohibited on any vehicle other than a law enforcement vehicle as defined in WAC 204-21-020.
(5) Flashing white lights are prohibited on any vehicle other than authorized emergency vehicles, law enforcement vehicles, school buses, and emergency tow trucks as defined in WAC 204-21-020.
Punishment
Penalty for violation of vehicle lighting regulations is a $124 fine.