Arteis,
I did not overpaint mine and would not really consider it because they look fine. However, if I did not like the color I'd be reluctant to overpaint. A thought that just occurred that I might be willing to try is using watercolor pencils - with a dulled tip. I know that will be more labor intensive but I'm betting safer. Painting the type of paper mine is made from would likely result in either ruined stuff or curled edges that would be hard to fix. Perhaps what you have is sturdier. I think mine are from either Noch or Preiser if I recall right.