You could apply colored squares of tissue, like they did on Trading Spaces. It looks cool and retro!
Or you could staple up fabric, my mom did that, it looks nice!
Or put tons of posters all over...
If you want to do any sponging or other faux finish on your walls, try it out on a plain sheet of drywall first. Drywall is very inexpensive and the home improvement stores near me will even let you by half sheets or scraps. Also, Home Depot sells returned paint at a serous discount. Look to see if there are any colors you like. You can always have an additional can mixed to that color if they don't have enough returned paint to do the whole project.
Hope this helps.
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