I posted a topic on this a little while ago. You can try Conceal from the makers of TiPaint. It costs $20, and if you try it I'm sure a lot people around here would like to know how well it works, so post a review!
To be honest I didn't really try. It took about a month for the stuff to arrive and when it did i only used a tiny bit. Overall it was much to strong a colour. It didn't look right so I just left it alone.