Do you have experience with greenfoot? If not you should first read the API (not everything, just the important things) and try it out, so that you get a feeling for it. If you know how the circuits work you can program the logics so far, but for the GUI, I will help you. I don't want to give the whole project away, but I can give you some of it, so that you know how it works.

It would be nice to be able to paint with different colours and Images at the same time, so that not all what you've painted will be changed.