I don't know a whole lot about these. I first spotted them over on a Pinterest board all about mail. (It's a nicely linked board and quite well done. Too often, I find Pinterest to be a land bereft of citation.) A bit of Google image searching lead me to a site selling folk art from India. There is a tantalizing bit in one of the descriptions that states that the stamp paper doesn't have chemicals in it that interfere with the paints. Whatever the reasoning, I think these are fantastic and thought you'd like to see them too.