Making a map in GIMP

[CAgrippa]

Possibly too late on this, but if you create a new layer, then in the original image do Select > Color, you can select one color all through the image. Simplest thing is to fill that black or white in the new layer, then select another color the same way, fill it, and then you have as many sections as you want all colored the same base color. The new layer can then be used as a mask -- just alpha-to-selection it, invert the selection, and fill with white or black (the opposite). From that point, you can pretty much select or create as you like. Hope that helps.

[EDIT:
Oh: I just realized that you're looking to select a RANGE of colors. Oops!

If you create a new layer, and select by color the outside edge colors, then fill them with black, you should have a black-outlined thin strip containing everything in between. Invert, fill with white, then use the wand to select the white area inside, and color it black. You should now have the whole area black. You may get little dotted lines here and there to paint out, but this is quick. That whole black area can now be colored as you like.

Hope that works for you.