In the days of the darkroom this Photo Masque, for example, could be brushed on top of black-and-white photos that were processed in your own darkroom. Brush it on areas that you wanted to protect. Then, once this red colored frisket dried the print could be placed in, say, a blue toner. Once the toned print was to your liking the frisket could be removed without damaging the emulsion. The red frisket prevented any blue toner from reaching the emulsion. In the end you would have a uniquely finished photo.
Today when thinking of Photo Masque or Photo Mask it’s easy to associate this with Photoshop’s Quick Mask Mode shown in the example below. Instead of dealing with the potentially dangerous liquid version a protective red mask is painted on a digital image. (When we mention that it’s dangerous look at the back label shown in the image above.)