I guess I wasn't all too clear on what I was asking. I am a firefighter, I currently use a green light, and when we get a call I respond to the station. I have a new light for the front of my vehicle which leaves me with a spare. What my question is, is instead of putting the light in the garage or getting rid of it, would it be legal or not if I were to just mount it in the rear window to use on the odd chance that I roll up on an incident. It would not be used as a response light. The reason I ask is that we can leave our lights on at a scene if we are there first, but it is not effective to traffic approaching from the rear. I'm not looking to go out and buy an amber light, and I'm not needing or wanting a rear facing light with any sort of passion, it's just a matter of me having the light, and if I can put it to use, great, and if not, oh well.
It only applies to rolling up on an incident. If there are any services on scene already, I wouldn't even think about using it. It would probably get used maybe 5 or 6 times a year.
Additionally, we do not have green lights on our command vehicles, so there is no issue there.