Don't know if you're aware but there are a few permanent inks for fountain pen that are available. They are not india or acrylic inks (those will kill any fountain pen) but real fountain dedicated fountain pen waterproof/water resistant inks. That's what I use for my watercolors. I can suggest:
- the German De Atramentis Document inks. That line, specifically, Quite a few colors are available.
- Platinum Black (to get a superb very black black)
- I've also had good experience with Rohrer & Klingner Sketch ink.
They are rather flat inks (I remember you mentioning that in other discussion), the flatter being DeAtramentis and Platinum, but Platinum still feels much smoother. They all work fine with my a Lamy Safari F or EF.
BTW, Platinum Black is the same brand as the Platinum Preppy cheap fountain pens and they work together flawlessly which makes them a great duo four outdoor use ;)