Redrobes - that does work, but the problem then is if you decide you want to edit the colours and leave the black line in place. You've got the colour and the black lines on the same layer so if you erase one, you erase both.

From the maps I've seen, the backgrounds don't seem to show through the colours. Do you just keep them for the edging of the image - or do you let them come through the colours a bit? It look a bit like they are 100% opaque colours under the ink layers.