These types of laws aren't set at a Federal level in the US, but rather, state-by-state. As far as I know, they all allow red or amber. Most US cars have red turn signals.
The red comes from the fact that combined red turn signal+brake lights with white reverse was the standard for a very long time, from at least the 1940s.
Interesting, thank you. Since I've never been to NA I just took the yellow turn lights for granted.
I just looked it up for Germany, and apparently turn light have to be yellow for every newly registered car after 1970.
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.
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.
