I’m a bit late to the game, but 53 paper really is a delightful drawing app. I can’t say I’ll give up my paper notebooks completely (they are my lifeblood), but the watercolor is an especially lovely feature, it blends and layers beautifully!
I was put off by the limited color palette at first, but after playing with it for a bit I found I could get a pretty full spectrum of colors.
The basic app is free and each of the tools (watercolor, pencil) are 1.99. The format isn’t particularly useful for working with illustrator, so this is really just a casual drawing/note taking app. Overall, I would recommend 53 paper as an intro drawing app or for those who want a low key but high quality note keeping app!