I have a Retro 51 rollerball in which I keep a red ink refill. I don't use red ink often enough to want to keep it in a fountain pen, although I use them for most other things.
Looking inside that pen just now, I find that it has a Schmidt P 8126 refill. So I think you'd be safe with that, and of course it's not just available in red. Going to the Jet Pens site, (disclaimers, not affiliated, link is for information but I have bought from them) and looking at the recommended refills for these pens confirms this. It will also take Schmidt EasyFlow 9000 ballpoint refills and a number of other refills with the Parker ballpoint form, even though the end caps on those are a quite different shape from the Schmidt/Retro 51 rollerball refills.
Edited by ISW_Kaputnik, 29 July 2020 - 23:20.
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