Possible reason for the Ribbon to show up as gray color is:
Excel is running in compatibility mode.
1. Browse to the following location: C:\Program Files\Microsoft Office\Office 12.
2. Locate the file excel.exe.
3. Right click on the file and then click on the properties option.
4. Click on the compatibility tab.
5. Uncheck ‘Run this program in compatibility mode for’ checkbox and click Ok. Try to open the excel file which had color issue and check the Ribbon color.