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.