Set up your scene and make a normal render.
In the compositor, use a masked displace node and a masked color correction node.
For the mask, I used…
https://www.textures.com/download/overlays0083/138041
Final watercolor render:
If you have a different method, feel free to post your own tips to teach people your technique for watercolor renders. 
The result is great, but isn’t there any free texture that can do the same job?
There’s a lot: https://www.google.com/search?q=watercolor+texture&tbm=isch&hl=en&tbs&rlz=1C1ONGR_enUS984US984&sa=X&ved=2ahUKEwj5v4PpzcH2AhXgm2oFHX9JAg0QzKUFegQIARAb&biw=1901&bih=977 you’ll need to check the usage rights of any of those of course, but there’s a bunch
Thank you, I’m searching for the best way to get a painted shader effect for this project:
https://www.z-anatomy.com/
If you feel like playing with it and propose some improved shader, it is welcome.
The one shown at the moment is based on the Comic chader from Paul O. Caggegi.
should do it, but maybe that your compositor trick is all I need, actually. It is much simpler.
The textures ARE free. After creating an account, textures.com gives you 15 free texture credits a day. The texture I used only costs 1-5 credits depending on the resolution. The terms of service states, " Content downloaded may be used for personal and commercial use (provided the use is in compliance with these terms and conditions)." The only reason to buy a subscription is if you need more than 15 credits a day. The subscription I had for about a year was $14/mo for 1,000 credits a month. You can get 1,000 1K textures a month with that. I would advise you to not get the studio subscription, unless you want to share textures with your team, get access to SBS source files, or get access to their 3D model library.
https://www.textures.com/about/terms-of-use
Although, one complaint I have about textures.com is that they don’t allow you to package their textures in 3D models you sell because that counts as redistributing their textures. Using their textures in renders, like to make a watercolor effect, is fine. But if this is a deal-breaker for you, then I’d recommend either making a procedural watercolor texture, or using a marble texture from ambientcg.com (CC0 1.0), or find some public domain textures on opengameart.org/textures.
I thought that mixing it with this one should do it, but maybe that your compositor trick is all I need, actually. It is much simpler.
The compositor makes it look more like real brush strokes because it moves the edges around, but I’ve seen people do realistic watercolor with alpha materials that make it look like a plane is a brushstroke or something. 3D models have straight edges, but watercolor has less defined edges.
