What colour of paint should I use?

You don't want to put bright paint on all the walls, it would be too distracting and no one would even pay attention to the bedding. I like the idea of three walls one color, like a like brown to match and then a bright solid green or blue on the other wall. Just remember that colors get brighter once dried. I would go with the light brown and then green or blue- whichever your boys like better! Make it a fun project with them and they get to choose! You could even do green on one wall with a blue chair rail border! Good luck!