I can be certain, you cannot legally have Amber running lamps to the rear. It was the introduction of foreign cars that allowed amber to the rear at all. It's DOT lighting law that amber is reserved for lighting and reflection forward of the halfway mark of the vehicle and Red to the rear of the halfway mark. Amber is permitted as a turn signal only (And flasher) at the rear. http://fmvss108.tripod.com/
(more info on lighting)
This link is for everyone but the people who live in USA. STATE LAW links.
The law concerning this i believe would be.
see S5.8.1 of Federal Motor Vehicle Safety Standard No. 108).
Another informative site on lighting including aux lighting.
You want to see how anal they CAN get?
The laws are very specific is you look at the links, material, color, brightness, location, height, width apart, it gets very complicated. And quite a many Motorcycles alter lights and make them illegal, No one will ever have a problem unless they have an accident that seems to be someone else's fault until some hot shot lawyer points out the lighting deficiency, then.......... there goes your settlement and now YOUR at fault.
BUT! To add a Twist to the subject, it's fact that like 10% of male population can't see red, and some states are considering to allow amber to the rear as running lights, (When next to red running lights) go figure. I have no idea if it is true but it's rumor that a 99 to o1 COBRA had amber running lights.
All in all, It's a good chance Amber running lights can get you stopped and a ticket, and your fellow motorist by habit are expecting to see Amber to the front, red to the rear, white and headlight, and from the side Amber and red in proper location show direction of travel. So Keep em how people are used to seeing them and your better off.
In my few million miles of travel, I have to say to me Flashing Amber in Fog is the MOST visible.