Amber rear turn signals are an utterly basic component of a proper automotive lighting system, it's shameful to treat safety devices as stylistic toys, but having just returned the other day from a large automotive lighting technology congress in France, I can say you haven't got the whole story. I spoke with the BMW exterior lighting chief, and he told me they wanted to use the same type of amber rear signal on their US-market E90 3er as they use everywhere else in the world, but the US DOT objected. This type of "hidden" amber rear signal uses a clear bulb with a green plastic balloon over it, all behind a rose-red (dark pink) lens. The result is a lamp that looks red when off, but shines amber when on. It's subtractive color mixing. The technique has been in use for about 13 years or so -- no problem, such signals work fine and don't notably degrade with age or use. But, the DOT said the green plastic for the balloon was not on the list of approved plastic materials for use in car lights, and they kept saying "no!" or simply stalling and not answering at all regardless of how much test data BMW submitted. Finally DOT said "OK", but by then it was too late; in order to meet production schedules, BMW had to make an alternate choice for the US market. They could've gone to a plain amber lens, or a clear lens with amber bulb, but for whatever reason (probably related to tooling cost), the red lens was their emergency "Plan B". This is the same reason why the first-year Audi A8 had these green-ballon/pink-lens/amber-light rear turn signals, but subsequent years had red lenses: DOT *****ed about the green balloons being made out of an unapproved plastic.

The situation is different with Audi: On some of their present car designs, they couldn't have an amber rear turn signal without the red brake/tail lamps being too small to comply with US surface area requirements, so the only choices were to redesign the rear lamps entirely (larger - not approved by the stylists) or have a red rear turn signal for the North American market. This is the same reason why older BMW 5er wagons had red rear turn signals. Other Audi models have plenty of rear lamp area to have an amber turn signal, but as Audi's chief of exterior lighting explained, "if some of your vehicles have red and some have amber, this does not look like a coherent line of vehicles. I don't like red rear signals, but they're just as legal as amber in North America."