Yeah, I was looking for a good CMYK fountain pen ink set. Nobody seems to make such a set. I could get a lot of half-solutions that would kind of work, but nothing beats the colour space coverage of a complement of CMY inks that were specifically designed to cover the whole colour space. They’re also about 10 times cheaper than fountain pen ink.
(And I got my printer ink for free on top of that from a print shop that just discontinued sales of their manual printer ink refills. The shop was Prink in Oulu, Finland. They probably still have these free refills.)
About six drops of ink and water the rest of the way gives you an entire cartridge of ink. This stuff is super concentrated.
I would use printer ink for the K too, but that’s too much of a crapshoot. Too often, the K is pigment-based, and that is likely to ruin a fountain pen. And it’s easy enough to find a good neutral black fountain pen ink. That is what the Platinum carbon black is for. It’s actually even more concentrated. Just one drop of it divided between two refills makes about a 50:50 grey that I can further modify with the printer ink. For less grey I have to go all the way down to one drop every four refills.