Back in the days when I was younger and foolish enough to go into art supply stores without a minder, sometimes they sold "scrape" kits. The idea was there would be a pretty layer underneath– rainbow, foil, etc– and a black layer above. You'd use the enclosed blunt-yet-edged tool to scrape away some of the black and selectively reveal the pretty layer. The resulting effect even with the kiddie-tier kits really impressed me.
But kits cost money and get black debris everywhere, and also I'm bad at that kind of linocut-style reversal of positive and negative space. So I figured out how to make digital ones! Luckily it's dirt simple: