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.