the sidecar probably isn’t that useful here, most of this is done in GIMP.*
edit: depends what you want to try and imitate though, looks like some of his prints have some kind of hand drawn element to them so you can approximate those lazily with G’MIC filters too:
thanks, I’d been pondering today if there was some way to give the colours a “hand painted on” look, easy in something like Gimp I suppose, pick the colour and overpaint the edges with it, or maybe something like blender with some node based compositing, 2nd copy of the image for the outlines blended with the original
It’s easy enough to do in darktable with one or more masked instances of color balance rgb. You are mostly limited by what you can do with the masking. It’s definitely something I will explore some more.