Rear-mounted turn signals in the States can be amber or red, at the discretion of the automaker. I’ve always leaned toward amber, mostly because, well, there are enough red lights back there already, but while my current ride (which has amber lights) was in the shop, I noticed that the car they lent me for two days had the red ones.
I’m not sure that this matters a whole lot. A 2009 NHTSA study of vehicles that had switched from one color to the other [pdf] suggests that amber is a tad better at reducing rear-end collisions, although it seems to me that this data will inevitably be skewed by the fact that generally you have to slow down to make a turn, and brake lights are always red.
And this presents another question: is it better to have a colorless bulb and an amber lens, or an amber bulb and a colorless lens? (I have the latter.)
Outside North America, the turn signals are always amber, which tends to suggest that Detroit likes red because it costs less. I don’t know about that, though it seems that you can’t actually get red bulbs: it’s always a colorless bulb behind a red lens.
Do you have a preference one way or another? If so, why?