After buying like 40 bottles of different inks and being unhappy with every last one of them for daily use I finally decided to mix my own ink. It is a 1:40 red-black pigment ink. One part platinum carbon ink to 40 parts platinum red pigment ink.

The Platinum pigment inks were the only ones to provide consistent flow to my ultra flex writing while still being permanent. Best yet it's more permanent than iron Gall inks. Sadly they only had 4 colors none of which I liked for everyday use as I prefer red-black inks. So I just mixed my own and pretty happy with the color. May increase the ratio of black slightly next go around though.