New York State: Affixed Vehicle Lighting Colors and Their Meanings

Blue Lights

You're driving home from work one day on the Long Island Expressway when a car pulls behind you with a flashing blue light - what do you do and what does the blue light mean? A blue light, according to the New York State Vehicle and Traffic Law, a "[...] blue light may be affixed to any motor vehicle owned by a volunteer member of a fire department or on a vehicle owned by a member of such person's family residing in the same household or by a business enterprise in which such a person has a proprietary interest or by which he or she is employed [...]" (New York State Vehicle and Traffic Law, 2007-2008 Edition). A motor vehicle owned and utilized by a volunteer fire department member affixed with a blue light is also typically known as a "POV", or Personally Owned Vehicle. Written consent from the Chief of their department is typically needed to affix such a light. However, an affixed blue light on a volunteer fire fighter's vehicle doesn't make designate it an "emergency vehicle", meaning that the blue light is more of a courtesy light, so you don't have to move.

Blue lights are also seen in the back of other emergency vehicles such as first responder vehicles, police units, ambulances, and fire apparatus.

Blue Light Conclusion: Non-emergency, volunteer firefighter personally owned vehicle. You do not have to move for this vehicle as it is a courtesy light.

Red Lights

Red lights are the big emergency vehicle light in the state of New York. All police officers, sheriffs, fire chiefs, first responders, ambulances, fire police, and fire apparatus will almost always have at least one red light affixed to the front of their vehicle. Red lighting on a vehicle, in the state of New York, signifies emergency vehicle status, meaning that you're mandated to move. Find tips for driving an emergency vehicle here.

A Red lights may be displayed also when fire apparatus is returning from a fire alarm or other emergency call.

Red Light Conclusion: Designated emergency vehicles. Either you're being pulled over are you should move to the right to allow the emergency vehicle to pass. May be affixed on police, fire, and EMS units and are accompanied by sirens.

Amber Lights
Amber lights are generally used for rear projectory in emergency vehicles. You'll see amber lights on the back of fire apparatus, or on arrow sticks affixed to police cars and other emergency vehicles. However, amber lights also have other uses in the state of New York - they can be affixed to construction vehicles and private security vehicles.

The New York State Vehicle and Traffic Law also states that it will "[...] not prohibit the temporary affixing and display of an amber light to be used as a warning on a disabled motor vehicle or on a motor vehicle while it is stopped on a highway while engaged in an operation which would restrict, impede, or interfere with the normal flow of traffic" (New York State Vehicle and Traffic Law, 2007-2008 Edition).

Amber lights are also found on tow trucks, whether it emitted from a dash light or a full lightbar on top of their truck.

Amber Light Conclusion: Amber lights are for construction, tow trucks, and private security vehicles. They may also be used on disabled or stranded motor vehicles on a highway. Also, amber lights are widely used for rear projection in emergency vehicles and on arrow sticks to help direct the flow of traffic.

You don't have to stop when a motor vehicle with an affixed amber light (or lights) is behind you - the lights are merely cautionary. However, when used by emergency personnel or construction to direct traffic take caution and follow the flow of the directional arrowstick.

Green Lights
Green lights are essentially synonymous with blue lights, except they are affixed to vehicles being used by volunteer EMS members. These are courtesy lights as well. Green lights are rarely seen in the back of authorized emergency vehicles.

Green Light Conclusion: Synonymous with blue lights, except that they are affixed to vehicles being used by volunteer EMS members. Merely courtesy lights - you're not mandated to move out of the way.

Purple Lights
Purple Lights, although rarely seen, are affixed to hearses or funeral vehicles to warn others of a procession. These are by no means emergency vehicles, and you're in no way mandated to move out of the way for these vehicles.

Purple Light Conclusion: Non-emergency, used on hearses for funeral processions. You're not mandated to move over.

Ending Note
As you can see there are a wide array of lighting when it comes to vehicles. As I stated before, states vary on what their lighting colors are defined as. Look into your states Vehicle and Traffic Law for further clarification for your state.

Source: "New York State Vehicle and Traffic Law". 2007-2008 Edition.