In this (zoomed) picture you can see white artifacts at the edges of the coloured sides.
This is caused by me selecting the white area of each face, and using that selection to limit the colouring-in.
The problem is that the "magic wand" selection tool gives a selection with hard edges.
Feathering the selection almost solves the problem, but the fill gets noticeably lighter near the edges.
I am happy to use Gimp or Krita (or anything else available for Linux which supports Wacom Intuos tablets).
I think that what I want is a magic-wand tool which creates a selection with opacity equivalent to the difference between the white background and the current pixel.
Is there a way that I can draw lines and colour-in within those lines?