Determining percentage of selected area?

[jent]

I have a map of a country which includes the dividing wall between the two parties to a territorial dispute. I was wondering if I can use an image editor to somehow select the territory itself, and then select the corresponding areas of control for each side, and then get a breakdown of what percentage each side controls. Does anyone have an idea of how to do this?

 

[chown33]

Color one side solid red. Color the other side solid blue. Show histogram. The numbers of red and blue should match the pixel-counts of each, since that's what a histogram is.

Could also use a gray-scale image, color one side solid gray, the other side solid white, and do a histogram on that.