Colours on the screen - open to bidding

I need a simple iphone app based on an idea. What I need in a nutshell: if I put my finger or palm on the iphone screen, I would want to chose a colour (any colour I chose, or changing colours) that would appear on the screen under my thumb, palm, or whatever I put there, but would be blank/black on any other parts of the screen. Can you please advice me about the time frame this would take? I need a good product to upload to the app store and start to market. Cheers