Something quick for Adurna-Brisingr. I will put steps here, but I thought I might as well make it an editable. CX Sorry for the bad writing, it was just quick.
1. Put a streak of the colour on the base BG.
2. Add a bit of that colour with airbrush.
3. Add a bit more colour.
4. Add a bit more colour.
5. Add a bit more colour (lol).
6. This is my own personal opinion, but I like to go over the whole thing with pen at a low opacity, to clean it up. :3
There you go, simplez!