Amber turn signals, at least in the US, are not mandatory. It seems that import cars have them. Domestic cars typically have red turn signals, and now I'm noticing more import cars with them too. In fact, for stealth driving, I think red turn signals blend in better with traffic - someone a mile away will only see one red light blink and it could be a brake light or a turn signal. An amber blink means a turn signal only.

Since amber normally means some kind of road obstruction like a flatbed or tow truck that is not law enforcement (blue) or the back of a vehicle or an emergency (red), it should get you noticed.

However, since the bike is relatively invisible (versus, say, a flatbed), an approaching driver as absolutely no point of reference to judge where you actually are. They may run right into you thinking that little bright light is 50 yards up the road (or off the road if the road curves left). In heavy fog I've mistaken the back of an 18 wheeler's trailer (with its two pairs of red lights) as being two cars 4 lanes apart, when in fact I was about 50 feet behind a slow moving white trailered truck (and approaching it rapidly). An amber only taillight may be misleading regarding your actual position.

I'd consider putting amber lights pointing sideways, but for the rear I'd use red.

As a side note, if you have some extra money, I'd get the Down Low Glow lights. They resemble those cheesy under-carriage lights from Fast and Furious and related cars but they really, really work. I got blue (I have a blue kit so sue me ) and cars would actually slow down from 45-55 mph to 20 mph to check out the lights. I'd have a 4-5 foot wide halo of blue under me, my front tire and rear wheel (lights mount on downtube and left chainstay) were glowing blue. As a bonus you can see what gear you're in. 1 hour for 2 lights, 2 for 1 light, or something like that. Not cheap though.

hope this helps,
cdr