Most email marketers around the world are aware of one thing: it's quite impossible to have your email render properly and identically in all email readers, browsers and webmails...
Among the tricky rendering issues Outlook 2007 and Gmail stand out with their inability to display properly background images.
This triggered the following best practice: if you are ever to use white or some other light coloured text over a dark background image, always double this with a dark background colour so your text can be seen regardless the image shows up or not.
The guys at CampaignMonitor came up a couple of days ago with a solution to make sure background images are displayed properly in both Gmail and Outlook.
To make a long story short, they advise that you use two different ways of displaying the background image (each being one of the readers friendly) at the same time.
Please note this will not solve ALL the background image display errors (Lotus for example will still refuse to display them) so I advise you to continue to set a background colour but it's a nice trick you might want to try out.
I haven't tested it myself yet and don't know if it might have any side effects on other browsers/readers or if it can trigger any risk on a deliverability level, best thing is that you try for yourself.