Overall its looking really great, and I'm glad to see that you aren't going OTT with the actual colours.

However, there is a small problem developing.

Can you see how the paint on the overlying layers is starting to make the lines of the map look greyish in places? The green of the land is colouring the lines of the map so that they are starting to fade out quite a lot.

Try this for a cure:

Save your GIMP file, and then save it again under a new name "somethingorother version 3" or whatever, and try duplicating the map layer and moving the duplicate up to the top of the stack of layers, then changing the mode (numbered 1 in the tiny screen shot I gave you above) of that new map layer to "multiply". See if that sorts it out so that the lines go back to being nice and crisp and sharp black.

The colours you've already set up on the layers below it should shine through the top map layer as good as new, but they won't be colouring the line work any more.