Update cheatsheets

ppmcolormask

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# ppmcolormask
+
+> Produce a mask of areas of a certain color in a PPM image.
+> More information: <https://netpbm.sourceforge.net/doc/ppmcolormask.html>.
+
+- Produce a mask of areas of a certain color in the specified PPM image:
+
+`ppmcolormask -color {{red,blue}} {{path/to/input.ppm}} > {{path/to/output.pbm}}`