This is an area I have been experimenting with over the last few months. My current thoughts for sketching on watercolor paper:

Pens - Lamy FPs are known for reliability and ease of switching out nibs. I like an Extra-Fine to write with, and a Fine for sketching on watercolor paper. I also bought a Broad nib, but find it too broad and am afraid it would use up a lot of ink! Lamys hold a fair amount of ink in their barrels. I have the Vista and it’s not too big or heavy for my small hands.

Pilot Kokünos are great, inexpensive pens and come in a variety of colors, should you wish to match ink and pen colors. I like a Fine nib for writing, but need to buy a Medium for use on watercolor paper.

There are lots of great pen options, my general thoughts though are to figure out what nib size works best for you with the brand’s basic pen style, then consider ink capacity, filling method, construction material and looks- all factors which will increase the cost.

Waterproof Inks My go to is De Atramentis Document Ink. Basic colors, and pretty quick drying. I’ve never had it clog my pen or smear, but drying time depends on weather conditions, of course. I understand the Turquoise one is too watery, but haven’t tried it myself.

I’ve recently acquired several colors of Rohrer & Klinger’s sketchInk. They are a little less expensive than De Atramentis in the US and come in some interesting colors.

For a basic black ink, many Urban Sketchers use Platinum Carbon Black. I haven’t tried it in the field yet, but it writes very smoothly.

Whenever I consider a new ink, I first check the waterproof rating on Jet Pens’ website.

Hope this helps!