Its not a colour change. Hoel has already done all the work.
All I did was put a gray layer over top and adjust the light values a little. Here's another modified version and the gray overlay map beside it.
Personally, I like to add a little noise to maps so that I escape the sense that everything in it is planned. In my own works I find a little chaos artistically applied is usually a good thing.
Here's another version of the map with the modification layer beside it. Its not about colour, its about light. The differences are sometimes really subtle. Its much cleaner to add them as a layer gray tones rather than darken\lighten the original.
This is far from perfect. Just a quick thing to deepen the surrounding country.