@Arnd
Most fountain pen inks are not waterproof as they are made of dyes. They are made for writing. And because they are made of dyes, they will fade eventually with exposure to light. Even black will fade.
There are of course waterproof fountain pen inks but you have to read the bottle's label carefully. Such inks are usually pigmented and the purpose of using such inks for writing is for archival purposes, where you need the ink to last.
The inks in this articles are made for artists to make art. Typically artworks are for display, under light, hence they need to be pigmented so that the colours will not fade.
@Arnd
Most fountain pen inks are not waterproof as they are made of dyes. They are made for writing. And because they are made of dyes, they will fade eventually with exposure to light. Even black will fade.
There are of course waterproof fountain pen inks but you have to read the bottle's label carefully. Such inks are usually pigmented and the purpose of using such inks for writing is for archival purposes, where you need the ink to last.
The inks in this articles are made for artists to make art. Typically artworks are for display, under light, hence they need to be pigmented so that the colours will not fade.