If it's a combo brake / turn, (red). If it's turn only (amber). Amber is brighter, more eye catching. Otherwise, seen better.
If I saw amber lit up constant, I wouldn't think they were brake lights. JMO
That's why the only time you see amber lit in the rear is blinkers.

 

Just changed out my rear lights to all LED and couldn't be happier... Now when I stop, it really stands out. These are all red by the way, also changed out the tail light as well. Several have said the bike "stand out" quite a bit more, and when I step on the brake it is "VERY" bright.