One of the most popular external cosmetic modifications on cars is the addition of neon lights under the chassis, commonly called underglow lights. As with anything car-related, there are a slew of laws that pertain to lighting on cars. Most jurisdictions restrict the lighting that can be placed on cars to function lights such as headlights and indicators, while other allow for cosmetic lights under a certain wattage.
Most laws regarding motor vehicles are written in regard to operating the vehicle on the roads. While some jurisdictions have pretty strict laws regarding any modification, the general rule is that underglow lights are perfectly legal to have on if the car is parked but must be off while driving. This allows the owner to modify their car as they please and show it off, all the while maintaining the safety and limiting confusion on the roads
If your jurisdiction does allow for the use of underglow lighting, it is important to consult your local motor vehicles agency or police department as to which colours are allowed and which are not. It is a fairly safe bet that blue, red and white lights will be outlawed, as these are the colours of emergency vehicles. Displaying these colours is usually against the law even if they are on only when the car is stationary. Blinking and alternating colours are generally illegal as well.
Getting ticketed for an illegal car modification such as underglow lighting is typically what is referred to as a "fix it" ticket. Rather than have you show up in court and pay a fine or do community service, these tickets typically allow you to simply correct the problem in an allotted period of time and provide proof of the correction. Once this is done, no fine or penalty is assessed. The ticket will still stay on your record, but it also typically don't get counted as a moving violation. While it's not legally so, think of these tickets more as code violations than something like a speeding ticket.