Kaleidopen!
CSP
I don't have much examples of how to use this pen, or how to explain it BUT, I will say…
Downloadassets.clip-studio.com

a simple color mixing brush and quick color sketches. It can be used as an almost hard brush but lowering the opacity by 50% works best.
This pack is free — download it straight from the author's page. All rights belong to the author; BrushCatalog links to the original source.
How to install: download the file, then in Clip Studio Paint, drag the .sut file onto the Sub Tool palette (or Sub Tool palette menu → Import Sub Tool). More details →