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.