The technique of inside or (better known as) reverse painting is quite old, though I don't know for how long it has been applied to things which may be classified as paperweights in the widest sense. There is another collection topic, however: do a Google search on [reverse painted "snuff bottles"] and you'll find beautiful examples for the application of this technique. I don't know much detail, especially not on how the colour is fixed, to keep it from peeling off with time - but it doesn't seem to be a problem.