I found a plugin that lets you set a different background color or image post by post. I doubt I'll use it much, but it does give you a lot of flexibility. I have checked it out only by changing one post's background from white to gray, just to see if it works. It does. I'm not vouching for its overall safety, though, since I haven't used it much.
http://blogtap.net/custom_post_background_plugin.shtml