Ok, since my experience in map making is equal to zero or close, this question and problem might seem too simple to most, but for me its quite complicated.

So here is my problem: How do you edit a map as in adding nation/country/faction colors on it, to delimit their extension on the world map, without harming the map colors. My intention is to add these country/nation colors to the map, but without blocking or covering the map terrain colors, aka forests, mountains, etc, just enough so that the nation color can be distinguished over the terrain colors.

What program is recommended and is there a tutorial that might help me?

Thank you in advance.