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”.
By choosing “Configure Color and Opacity”, you can
bring up a dialog that allows you to set these to values other
than the defaults, which are red at 50% opacity.