The GAU happened. Gimp crashed while saving. Therefore the original map data file has lost its worth as the most important layers are missing. So I'm thrown back at a copy of the file from Wednesday last week, causing a lost of 25-30 hours of work.
I was so close to kill Gimp, my computer screen and my laptop.

Now, on the other hand I managed to save the new settlement layers. I simply have to redo the whole countries- and borders thing, the whole thing with the brightened coastlines of the sea and a dozen minor things.

As much time went into try and error, I assume that it'll take me only 5-6 hours to reach the status quo ante. And, as Max supposed, I could try out new colours for the countries.