Someone blogged a great DIY on this concept here: http://www.addicted2decorating.com/how-to-make-a-custom-wood-air-return-vent-cover.html. Or if you are shooting for a more vintage feel here is another one: http://club.conservationgardenpark.org/2012/11/pinspiration-diy-vintage-air-vent-covers/. I think white would look best in your situation because you don't want anything drawing your eye towards it. The white will help camouflage the vent.

Thanks Andrea!

mostly, they are best kept the color of surrounding wall or floor. they are not something to feature.

Thank you. You're probably right. I have wallpaper in my kitchen and they are covered to match. Looks really good too