Originally Posted by
Gamerprinter
Your linked example is simply a vector shape given a solid color (green), its not even a border, it just has a different color than the background. Which begs the question, how many countries do you need to designate and if more than one, are they going to be in different colors? You could do this in an image editor as well (like GIMP or Photoshop), but it would be easier in vector (Inkscape, Illustrator).
I use a vector app called Xara Photo & Graphic Designer 9, while not free, its cheaper than a lot of commercial software and very powerful, though this operation you need is very simple to do. Draw the shape of your designated nation, and apply a color - that's it.
Since my land is detailed with photos or drawn textures of mountains, rivers, plains, forest, etc, so that all the land in a region/continent looks earth-like, when I place borders they are only outlines, sometimes dashed, sometimes other settings, usually also give transparency (so you can still see the detail behind the border) with the interior blank, so the land textures show through.