You could do this by recording a macro. First click 'Record Macro'. When the window opens, name your macro and also input your desired keystroke in the 'Shortcut key' box. Using the dropdown, you can also choose where you want the macro to be stored, either in the current file or in your personal macro workbook. Click OK then choose your background color. Then click 'Stop Recording'. Now, every time you hold down Ctrl with the keystroke you selected, the background color will change.