My recent message to my fellow iPad Artists " Folks..so why should you look into paper seriously? Here is one reason: it imitates not just the natural flow of colors but also natural "mixing" of colors. Going back to the basics of primary color ( red blue yellow) and secondary derived color ( purple green orange) you will see that paper53 simulates that behavior. None other apps do that.. The closest I could go is the water color brush blending in Artrage. However it still lags behind paper. These guys have done great job with the algorithm

tulips in Stanford Mall done with Paper by fiftythree