Federal law (FMVSS 108) permits red rear turn signals, either combined with the brake lamp or a separate turn signal lamp. Interestingly enough, they require amber color front turn signals because the white ones were difficult to see when the headlamps were illuminated. It's too bad they didn't use the same reasoning for the rear turn signals compared to the stop lamps.

There would be a huge backlash if they tried. Standard trailer connectors combine the brake light with the turn signal. (the brake like turns on both turn signals 100%) Many trucks just send enough wires (which are expensive these days) to run the trailer and tie the taillights off of those.

Thus to require amber to the rear means the standard trailer connector needs to be replaced, then some sort of grade period for old trailers, and - well I doubt I could think of all the complex factors required. In 30 years we might be able to do it.

If they did mandate it, then it would only apply to new vehicles and trailers.

You still have to deal with people with old trailers and new trucks, or new trucks and old trailers.

Trucks and trailers are each their own entity with their own papers in Germany. Perhaps this would solve the problem in the US too? When you have a new truck but an old trailer, you'd just show both papers when being questioned about it => everything is fine.

The paperwork isn't the problem, it is all the adapters needed.

And a diode on the old trailers would let them work with new.