After editing 'stamp' also known as brush shape or head, make a selection, at the bottom (depending on your layout) selection options, tap 'brush' to add as a brush. In brush settings under the 'head' section activate invert and color buttons.

I made this with my finger on my phone because I forgot a stylus. Ignore the sloppy 'trimming'. It is purely to illustrate that it can be done on mobile.