missed another

mixing palette colors on a palette

isn't as simple as Yuv, Yxy etc. color mixing, or other types

depends on the brush type, and where on the palette you are, towards the
edge you will have different colors than on the middle, colors will
differ along the edges of the brush, etc.

difficult, but important

might be easier if you temporarily let the mixing palette and brush be
vector types, then RIP it back to bitmap space afterwards

--
Dale