Editing Selections With Quick Mask Mode
When creating selections, double-click on the Quick Mask icon at the bottom of the Tools palette, to enter the Quick Mask Mode (or Press the Q key). While in this mode a red color overlay will cover the areas that are not selected, and the selected pixels will remain uncovered. You can paint with black to add to the mask (your strokes will show up as a red overlay), or paint with white to subtract from the mask. You can also paint with shades of gray to make semi-transparent selections.
To change the color or opacity of the red overlay, or to add the color overlay to the selected areas instead, double-click the Quick Mask Mode icon in the Tools palette to open up the Quick Mask Options window.