In printing, a mask overlay uses positive or negative images to correct common color and printing ink deficiencies. The images are created from one or more color-separation negatives and used in accordance with a given negative. 

Usually, mask overlay corrects excessive yellow and magenta color from blue and green printing, respectively. It either covers the layers in your image completely, partially, or with varying opacity levels. The white pixels show underlying layers, the black pixels hide them, and the grey pixels show them in varying amounts.

Mask pixels consist of 256 shades of gray, each showing a certain level of opacity. You can use mask overlays to fade between layers or create precise effects.