I know when we speced our engine about three years ago there was NO mention of light color in 1901, and we didn't have to sign a waiver for not having amber lights on the rear. As Roto-Ray stated, Amber is supposed to conveys a message of "Warning" or "Slow Down" but much like a traffic light, no matter what color you use people still go the speed limit (+/- 10 MPH) by the rig.
As for who requires abmber lights, the FAA does require amber rotators on vehicle that "normally operate on the runways or taxiways" (this comes from the mouth of a friend of mine who is an air traffic controller). Like Photo 5 said, they show up better in the fog. Not sure if this is the reasoning for the lights on the TAU or not, but that is the info that I have.
As for who requires abmber lights, the FAA does require amber rotators on vehicle that "normally operate on the runways or taxiways" (this comes from the mouth of a friend of mine who is an air traffic controller). Like Photo 5 said, they show up better in the fog. Not sure if this is the reasoning for the lights on the TAU or not, but that is the info that I have.