First winter with my 550i, temperatures have went down and I've noticed that exhausts comes out from the left tailpipe only. The left tailpipe is also significantly darker that the right one, which leads me to the conclusion that the there isn't much going out from the right tailpipe.

Is this normal? Does the right one only open up when there's higher air flow (i.e. higher speeds) or should I have it looked into? I drive in city traffic/short distances mostly.

I'll take a photo of the tailpipes tomorrow, so you can get a better idea of what I'm talking about.

yes, that is normal. The left one is open all the time. The right one only opens at "wide open throttle" If you look at the right side tailpipe, you'll see a vacuum controlled actuator that controls a flapper inside the tailpipe.
You can rig it so both ports are open full time, if so desired

Thanks for clearing this up for me :) I think I saw you and someone else discussing this actuator in another thread and I thought this might be the case, but I wasn't sure.