I spent this evening watching Stephen Berry's Youtube series and reading his posts on the AR forum. This lesson was particularly inspiring for me:
Not so much the layer masks, since I do that in Photoshop all the time, but the idea of building up layers of neutral tone before adding color is interesting to me. Really, his entire series is fascinating, as you can tell he comes from a traditional watercolor background and brings that knowledge to the digital medium.
I then spent some time trying out his various techniques on a test canvas. Here are the intriguing results, quickly modified by adding different watercolor texture overlays:
So the one in the upper left is what I achieved purely with the AR watercolor brush, knife, and sticker spray tools. The other variations followed Stephen's suggestion of bringing in a traditional watercolor overlay layer to help give some natural looking texture. He recommends bringing it in right from the start though, as it can affect how you lay down color if you can see the texture as you're painting. I can definitely see how the addition of the overlay would affect the tone of the painting!