I had an odd thing happen in one of my Access databases. On one of the forms, a bunch of the combo boxes were changing colour when they lost the focus. It worked fine in Access 2003, but the problem appeared when running the program in Access 2007.
To fix, open the form in design view and view the properties of the detail section (assuming that's where the controls are). There will be a property called 'Alternate Back Color'. It's likely set to 'No Color'. Set it to white, or whatever you want the colour of your combo box to be. That should fix the problem.