If you have downloaded some Microsoft Project templates from the Microsoft site, when you open up the document it will be default to ‘Use AutoFormat’. With this option enabled, you will not be able to change the fill color of the template. To disable it, simply right click on an item in the template and de-select the ‘Use AutoFormat’ option. Once this option has been unticked, the backgroup fill color option will be enabled and allowing you to change the background color.