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.