If by using you mean selected, then the only way I see is to
use a macro. If you just want the best way to color everything
but a specific area then I think you already have the easiest to
implement answer.
You can invoke the macro with an assigned shortcut,
with a toolbar button, or from a shape. More information
on installing and using a macro see
http://www.mvps.org/dmcritchie/excel/getstarted.htm
Sub gray_unselected()
Cells.Interior.ColorIndex = 15
Selection.Interior.ColorIndex = xlNone
End Sub
Obviously using does not mean the used range, but I guess it
could imply the current range Ctrl+Shift+asterisk
then you would use either answer accordingly.