There are two Quick Mask properties you can change by right-clicking on the Quick Mask button.
Normally the Quick Mask shows unselected areas «fogged over » and selected areas «in clear», but you can reverse this by choosing «Mask Selected Areas» instead of the default «Mask Unselected Areas».
Use «Configure Color and Opacity…» to open a dialog that allows you to set these to values other than the defaults, which are red at 50% opacity.