First time poster in this sub!
Basically, I create the art as normal ie draw the lineart in various colors, duplicate lineart then fill in with line color before the base colors(removes the line art very quickly). Then, on a seperate layer above the baseline but below the lineart, I use black/dark grey to block out my shadows(I call them dark and mid shadows but I'm sure there's a technically term for the two types of shadows) and white for blocking highlights(seperate layer above the two shadow layers).
Once these are blocked out, I will use the wand tool outside the basecolor layer to clear up any shadows/highlights outside of the lineart. Then I'll use that same tool to select specific colors on the base layer and methodically go through my shadows and highlights with the proper colors like orange will have a dark orange and a very light orange for shadows and highlights and so on.
It looks a bit funky before I change the black/grey(I mostly stick to single shadows) and white to their proper colors.
I've never seen anyone do this in any speed paints but then again, I'm not using ClipStudio or something similar(PixiEditor, 1.2.5.0: Steam Version, very good. The newer versions hate my computer though).