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Usually, if a car has a strobe light on top of it, we are meant to pay attention. We are all familiar with the blue and red lights of a police car or the red and white lights of fire trucks and ambulances. However, many of us are less familiar with a car with green lights on the roof. If you see a car with a green light on top, you could be in big trouble. 

What does a green light on a car or truck mean? 

The U.S. Department of Homeland Security
The Department of Homeland Security logo is seen on a law enforcement vehicle | Samuel Corum/Anadolu Agency/Getty Images

Probably the most serious car or truck with a green light on top is the Department of Homeland Security. Yep. The folks who stop terrorists and other major scaries.

If you see green lights behind you, you probably shouldn’t have taken that USB drive from the shadowy figure behind the CVS.  I’m not sure what you have to do to get the Department of Homeland Security on your tail, but if you find the boys in green behind you, you are probably in big trouble. 

While the Department of Homeland Security is a pretty big deal, there are some other more common reasons you might see green lights on a car. 

According to Reader’s Digest, other groups can have green lights on top of their cars. You might see the Department of Transportation, the fire service (often volunteer firefighters or first responders), and command post vehicles all use green lighting. 

Can you put lights on top of your car? 

border patrol vehicle driving along a fence line
A border patrol vehicle drives | David McNew/Getty Images

If volunteer firefighters can use green lights on top of their vehicles, does that mean you can? Like many other laws, the rules about putting lights on your car vary from state to state. However, in most states, putting green lights on your car is illegal. These laws are meant to keep people from confusing regular cars with emergencies or other vehicles under official duty. 

What do other colored lights on top of cars mean?

Red lights

In the U.S., anytime you see forward-facing red lights on a vehicle is meant to signal an emergency vehicle; Police, ambulance, or fire truck. Although in some rare cases, like in Texas and Michigan, tow trucks might have a red light. In Iowa, even hearses can have a red light during a funeral procession. The other examples of red lights on school buses when they are stopped to pickup up or deliver children.  

Blue lights

Outside of the rare blue traffic lights in Japan, typically, a blue light on a car means police. In the U.S., blue lights are saved only for law enforcement. While police might also use red or white strobing lights, there is always be blue lights in the mix. 

White Lights

White strobing lights are most commonly used alongside the red lights of emergency vehicles; white lights might also be used on some government vehicles like postal trucks or sometimes busses. 

Amber lights

As with traffic lights, amber or yellow lights often are meant to signal caution or to slow down. If you see a strobing yellow light on a vehicle, it will typically be on an oversized load trailer, garbage truck, snow plow, and maybe a delivery vehicle. 

Purple lights 

Purple lights, like green lights, are probably the most uncommon light to find on a vehicle. The only official use for a purple strobing light is for hearses and funeral processions. Although uncommon, it is customary and kind to pull over and put on your hazard lights if you see a funeral procession.