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Mask and Reverse (otherwise called Remask) are useful features. There's two ways you can get to it, pic related. Also, there's a slight difference in the terms in Pro because of translation error.
Change the Mask/Remask/Reverse color by choosing it, then right clicking on the box for Normal, just clicking for Pro. If it's on Mask/ON, you can draw anywhere except for that color and it's opaques. (a semitransparent 255 255 0 will not be drawn on if it's set as the color, etc) Remask/Reverse does the opposite- only that color and its opaques can be drawn on.
Hope it's useful. It doesn't work with watercolors, pen, airbrush etc, well though.