"just recently it was suggested that when Luke approaches Bespin that it should be later in the day because the sun is setting shortly after..."

Hmmm...I'd urge to you rethink it again, Ady. Even though logically it works, emotionally and metaphorically, the sun doesn't really start going down until absolutely everything has fallen apart for our heroes. Here, Luke is arriving to save his friends, and we don't yet know how badly that's going to go south. The more blue, optimistic, well-lit environments support that emotionally - the young hero is here to save the day. When it's getting to be clear that he's totally overmatched, and the other band of heroes is desperately just trying to escape from Cloud City with their lives, and then shortly after we find that one of them is simply not going to make it out, we start getting that terrific melancholy lighting, and the metaphocial sun setting on our heroes. I think you risk shifting the mood too early if you start sunset at Luke's arrival. Anyways, just a thought...