To change the color scheme in Office 2007, click the round MSOffice button at the top left corner of any Office program and click options. You will then see a drop-down listing the three color schemes.
Here are some screenshots of Powerpoint 2007 comparing the three color schemes [you can't edit the colors though]



Important: The color scheme option is available only with those Microsoft Office 2007 programs that support the new Ribbon UI namely Excel, Outlook, PowerPoint and Word 2007.
