Most likely you can use an amber light as a "caution" or "warning" light -- anything else is probably not legal for a personal vehicle or news car. Here in NY state emergency vehicles get red & white lights, volunteer fire gets blue lights, and volunteer EMS gets green lights. Gall's is probably the best place to get one -- it's where I have bought my green one for EMS, and most lights are available with an amber lens.
I used to have an amber light in my S-10 Blazer at work. It gave me just a little piece of mind knowing that my vehicle was obnoxiously marked as an obstacle when I was running around shooting an accident from the edge of the freeway nearby. Unfortunately my bosses decided that there were liability issues, and I had to get rid of it. After all, the TV station would obviously be at fault if some idiot managed to plow into my carefully parked news vehicle displaying a bright "don't hit me" amber light -- as opposed to our limited potential liability if the same idiot hits my dark car parked poorly on the side of the road at night. I will never figure out why "common sense" is anything but common, especially in TV news.
Personally, I think that all LIVE trucks should have an amber "ArrowStik" traffic director lightbar (or at least a couple of flashing amber lights) on the tail end, and it wouldn't hurt my feelings to have a couple of big orange cones to throw behind the truck, as well. It works for power company, cable company, phone company, and other trucks parked in bad locations -- why shouldn't we be a bit safer, too ?