This is wonderful. Had a quick look on the Mac here.
I'm going to go out on a limb here and ask if you could add a "screen" more too. So totally not a sprite based thing you've built, but something one could use to paint an entire screen for fun. An almost "Artists II" replacement for modern computers. You may have actually stumbled on a great solution there. Though then there's be patterns, fill, fonts etc... So I'll zip it on that thought 🤐
Couple of other thoughts.
Any chance of a persistent "bright" and "dark" palette, rather than only a switch? I have plenty of screen real estate, so a switch is adding a click I don't necessarily need to have if the whole palette was around.
Ink/Paper. Did you experiment with having those in the same place? A single row of colours. Maybe left click for Ink, right click for Paper. Then maybe a little icon inside the colour square to represent which is set on which colour. Top left for the chosen ink colour, bottom right for Paper?
Anywho. This really is wonderful. Thank you for sharing it.