Using a touch interface for naturalistic sketching/water-colour painting can be complex if the application has a steep learning curve. Having just downloaded https://www.fiftythree.com/paper I have done my first sketch. I was interrupted by my 5 year-old who instantly took over and showed me the intuitiveness of the interface. Zero explanation, it is the closest I have seen to a piece of paper and a few pens, pencils, water-colour box. Amazing!