When Han, Leia, Chewbacca, and C-3PO arrive at Cloud City, there are panels in the background that have a warm glow behind them:
By the time Luke arrives, however, after the Empire's trap has been sprung, this warmth has disappeared, and the panels are colorless:
This contrast in color illustrates the change that has occurred. At first, Cloud City seems like an inviting place, but once the Empire's presence is revealed, it becomes a stark and unwelcoming environment.