Every so often I return to trying to create a shader that mimics the wavy-edged, often monochromatic style of "Hero" by hwei. It's not quite watercolor, but has a little bit of that effect.
This latest is an Arnold shading network. It takes three colors (base, shadow and highlight) and a few user-controlled inputs (position, spread and overlap ratio) and runs them through a decent number of math and range nodes to create a set of intermediate steps on a color ramp. This in turn, is driven by surface luminescence (combined with the results of an ambient occlusion shader) and layered under Arnold's Toon shader. It seems, last I tried it, plugging the ramp directly into the Toon tonemap eliminates the fractal-based edge variations that I want. It can also be used with textures, though any hard edges in the image will remain.
Eventually I may post a tutorial, or even the shader itself, but I still have some things to test and clean up. In the meantime, drop me a note if you have questions!
Panther/snake model is Georges Gardet's "Drame au Desert" from ThreeDScans.com.