Convert it to grayscale and take 3% of the black or white. And then convert it back to RGB. And save as bmp. Or create a duaton from that picture. This may be easier because I probably wouldn't want to edit this image manually.
Duathon, is a graphic term for creating an image with only two colors. Triathon with three and so on. I think gimp can do it too, but I don't know where to look there. I do not use it.