Underglow is a type of car lighting that is typically used for decorative purposes. It involves installing lights underneath a vehicle, which can produce a variety of colors and effects. While underglow is legal in some states, there are others where it is not allowed. This often depends on how the law views it in relation to other types of car lighting, such as brake lights and turn signals. In general, however, underglow should only be used when the vehicle is parked or off the road.