I am thankful for the Pentel Aquash, which I carry together with a pan of gouache. I know I can’t paint worth flapdoodle but it is interesting to observe how fountain pen ink mixes with other media. On the Fountain Pen Network, there’s a good thread that explains dye versus pigment versus paint.
Writing with a fine nib over gouache is iffy; when the water evaporates, the gouache feels almost powdery, and there is a real risk of getting pigment particles in between the tines of the nib, which can impair it. I think I prefer layering the gouache over the fountain pen lines and squiggles. It does give more depth, especially if I use a waterproof Noodler’s ink in the pen.
Fountain pen ink also behaves differently when spattered with a brush – at least the Caran d’Ache Saffron I decided to have fun with. It just plops down and stays there, unlike watercolor spatters which can run.