I saw Kaetemi's post and video about applying watercolor to white, strong and flexible, and it got me thinking about using my Derwent Inktense watercolor pencils on it. Inktense are a little different from normal watercolor; once they are fully saturated with water and allowed to dry, they cannot be removed with water and are relatively permanent. They also offer some really intense, ink-like colors on paper when washed out with water.
In the attached photo, I have tested, from right to left: Copic Ciao Powder Pink (YR00) and Milky White (E51), Inktense Teal Green (1300) and Inktense Fuscia (0700) applied with a waterbrush, Inktense Sun Yellow (0200) and Sea Blue (1200) pencils applied directly and then wetted with a waterbrush and mixed.
The porous nature of WSF makes it act like paper; the rough texture acts like paper "tooth" and grabs the pencil and marker very well. I would still recommend a spray-on clear coat for protection.