Yes, as ravs says, you can click on the layer of, say, the forests, and then go to Image>Adjustments. Use Hue/Saturation, Brightness/Contrast, or Levels to play with those colors. (You can also use Curves if you feel like playing a lot; it's a beast to figure out, for me at least). If you don't want to do an entire layer, you can also surround things with the marquee tool to only mess with certain sections: surround whatever you wish, then Image>Adjustments.

Also remember that if you Layer>New Adjustment Layer you can also you can also do these things in a "non-destructive" manner--meaning the original maps' colors are in tact, with the effect applied over top. This is a nice safety net to use.

Make sure to keep us posted with posts!