You could get some removable wall stickers. i just searched on Amazon and there are tons of them, and they will peel off without damaging paint:
Cheap and cute!
Or you could add a giant, colorful poster or "tapestry" - Giant canvases are pretty expensive but if you are somewhat handy you could easily make wood frame and nail some funky fabric around the edges.
Ditto that adding color to furniture or maybe a colorful lampshade, rug or pillows would help.