Legos are all about creativity, so that's definitely the overarching theme for any Lego bedroom. (Don't panic! You can be creative!) And given the limited options for Lego decor, creative is definitely the way to go here.
To that end, there are a lot of ways to get creative with Legos. Not only can you use actual bricks within the decor, but they're fairly simple to paint on surfaces where using bricks doesn't work as well. Basic rectangles and simple circles and you're good.
Color scheme options for a Lego bedroom definitely include blue, green, red and yellow - the primary choices for Lego bricks. You might want to pick one to be the dominant color, then work the others in as accents. That just helps keep everything from getting too crazy and gives you some boundaries as you make choices. In fact, keep an eye on the mix of colors in general to ensure you don't get it overwhelming.