There are two QuickMask properties you can change by right-clicking on the QuickMask button.
Normally the QuickMask 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.