I created this crayon-style NPR render by giving each material a separate ID. Then, in the compositor, I dilated and blurred the mask a couple times, getting progressively smaller and lighter in color. This way, we ignore the shader entirely and create more of a watercolor effect where it's darker on the outside, and lighter in the center. The smaller the object or further away from the camera we get, the less likely the lighter bit will show up which is convenient.
Test it out by adding new objects into the scene and giving them the preassigned materials. You can create your own materials by copying the ones I already have and increasing the material index number (the color of the material is meaningless, it's just to make it easier to see in the compositor). Then just duplicate the nodes, change the colors, and combine them back in following the same pattern as the other ones.
For this blend, I used a ricepaper texture (http://maplerose-stock.deviantart.com/art/Rice-paper-texture-172001815) but do experiment with other textures as the results will vary dramatically if you change the background texture or blend mode (I'm using burn, but linear light and multiply as create great effects).
I'm also use Edge Nodes which come from Blender NPR (http://blendernpr.org/edge-node-v1-2-4-july-2015/) although I downloaded them as part of the Sharkigator node collection (http://sharkigator.deviantart.com/art/Sharkigator-Node-Collection-Rev-004-583309156). You might consider turning them off and tinkering with FreeStyle instead, textured lines can further the effect.
I've been a BlendSwap user for a while, I'm excited to finally be contributing something! Feel free to use this however you like, I'd love to see what you come up with, or if you find a better way of doing it.