The way I work on the watercolor ones, which is by all means not perfect, is:
1. Adding the two layers, one the character and one the watercolor background. I crop the character model out by using an eraser.
2. Choosing the multiply option for the layer merge tool on paint.net
3. Tuning the brightness and contrast
4. Then poster filter (Photoshop) or ink drawing in paint.net (not as good as photoshop's filter, but I refuse to buy photoshop ). How it comes out depends on the style of the drawing, sometimes the sharpen tool from paint.net helps add definition to at least give enough material for the poster filter to do its magic.
5. Then cleaning up with the stamp tool. Extra unwanted lines or over exaggeration of dark circles.
(1-3 I do cause it's the easiest for me in terms of matching the color of the character to the background)
There are some really talented ppl on this forum and I think previous pages in the potrait forum give some tips :} I learned a lot from trying to emulate their style and approach for my character potraits.
Thanks for the pointers, this is really helpful especially as I've been stumbling my way through pain.net (I'm no artist so I'm not investing in photoshop either).
I've been trying to apply these guidelines to the picture I've been using for my last guy in PoE:
I've actually found the original online: https://img00.deviantart.net/e235/i/2013/228/6/6/geena_by_omupied-d6iek8y.jpg
(I didn't know it was from devianart as I must have nabbed it from a portrait collection for Baldur's Gate)
In any case here is what I've managed to accomplish with paint.net (it's far from perfect and it's certainly a bit rough but it looks serviceable enough and will have to do):