Start with a line art drawn in black ink. Scan at 300 dpi
resolution in grey level. Load the image in GIMP, remove
unnecessary parts, and threshold.
Make a duplicate layer of the image. Change background in the
original image to transparent. Make it top layer, lock and hide it.
Colour each flat of the copy using flood fill (may need to tweak
the copy to help flood fill). Cover every bit of the copy with
flats. Now lift each flat to its own layer by
ctrl-shift-l. Verify by hiding copy and showing original. All the
flat layers must be sandwiched between orig (top) and copy
(bot). Always construct simpler and smaller flats first. The more complicated
ones are often best done by elimination.
Lots of pockets: Poke holes in the pocket boundaries with
the eraser. Make the eraser hard.
Shift-select all neighbouring colours as well as
black. Brush roughly.
Use desturated colura. To pick such colours, click on the S check
box in the colour picker. Now set S=50. As
you drag your pointer on the colour field you'll get only colours
with S=50.