that is called having a mask! here is the tutorial from the site: http://www.feral-heart.com/index.php?option=com_content&view=article&id=7&Itemid=10 (http://www.feral-heart.com/index.php?option=com_content&view=article&id=7&Itemid=10)
it's not for experts at all, anyone can do this!
This is Gimp's official website: Gimp (http://gimp.org)
To make your terrain two different colors, you need a mask, which you can make in Gimp. Simply just get two random colors that are bright and color in where you want them on the map. When you finish coloring it in, save it as (whatever).png to Feralheart >> media > terrains. When you go in game, go to map maker an open your mask.
(Random map mask)
(http://i59BannedImageSite/99nkpi.png)
Ok, so there's the map mask I have made in Photoshop (you'll be using Gimp.) When I enter it into Feralheart's map maker tool, I'll get it with a few different colors, or terrain types. The stuff in the red boxes is basically what you want to look at. (Note that I used three different colors for this!)
(http://i61BannedImageSite/67p8iq.png)
At the first box that is closest to the top, that's where you'll enter in your mask's name. Because I'm not actually using this for a map an just showing you, I named it "maptest.png."
Now that box underneath that first on is the terrain types you can choose from! There's plenty that you can use and experiment with. Now, that one bow of to the side with the "1" is very important when using different colors. That's how you change the terrain type of all of the colors you used in the mask. So, when I'm done and happy with the terrain I set for the red part of my terrain mask, then I'd click that little arrow next to the one which let's me change the colors for the blue. Here's a .gif I have recorded basically showing you: http://www.mediafire.com/view/t5k4am9yzb524sq/mapexpalme.gif (http://www.mediafire.com/view/t5k4am9yzb524sq/mapexpalme.gif) The .gif's file size was huge, so tinypic wouldn't take it (it might take a little bit to load.) That link should work.
I hope that cleared it up a little bit!~