@AnasaziWrites - All of the drawings I did for this year's Inktober used fountain pen ink instead of traditional watercolors, capitalizing on the chromatography they display when broken down by water. So no - not just for fountain pens! I use my fountain pen inks with pointed flex nibs, broad edge nibs, brushes, ruling pens, automatic pens, music nibs, pipettes, and toothbrushes! and fountain pens.
All my Inktober drawings are "monochrome" in that way, even if they seem to show different colors. I love monochrome drawings for helping me focus on composition and shading in quick sketch studies like the ones I've been posting. Dye-based inks are reactive to bleach, but not waterproof and hence not "permanent" like an iron gall ink or an acrylic or a traditional watercolor - all of which are pigmented instead of dye-based. But my drawings & paintings aren't so precious that I'm worried about them lasting through the ages.
When I use these inks with dipped pointed and broad edge pens, I improve the consistency for such work by decanting them into dinky dips and mixing them with gum arabic until they do what I want them to. But on good watercolor paper, many of them work fine straight out of the bottle - they just usually won't give you the world's most delicate hairlines that way. And they are highly sensitive to the quality and variety of paper you use!
My Dante project is going to use these sorts of inks & bleach on treated canvas, which is a huge leap into the unknown. I'm in the process of doing test swatches. The real challenge of using them for art is being open to the serendipitous results. You can't really control them like normal watercolors, but they reward experimentation and give beautiful results to those who persist.
--yours, K
