Large scale It stands to reason that if you're not allowed to paint a wall a color but really want a wall to be a color, find large-sized posters and canvases that will allow you to hang a lot of color, at the very least.
Let the art go overboard If you've got to stick to very white walls only, don't be afraid to find art that experiments with lots of colors and energetic patterns; this will heighten their impact.
Don't be afraid to paint other things Even if you're not allowed to slap some hues on the actual wall itself, no one said anything about painting frames or other hang-able accessories in the color you want. Combine that with bold and colorful art and folks won't even notice your lack of wall color.
Remember that white walls actually can be a good thing Why do you think so many art galleries employ them? They make art pop and will make your space seem clean, open and fresh, to boot. Don't see them as a handicap...see your whole space as one white canvas with which to use art as the paintbrush!
What do you think? Are you not fooled by bringing in color through just art and wall hangings? Or have you successfully pulled off this wall paint-less colorful trick?