30 water : 1 platinum carbon black : 5 yellow : 5 magenta is the mix for octarine.
But seriously, 30 water : 1 platinum carbon black : 10 printer ink is a good starting point for mixing. That is about how sensitive the mix is to each type of ink. Pure printer ink won’t ruin the pen, but it bleeds like a motherfucker. If you don’t care about the next five pieces of paper, though, you can do some pretty cool stuff with it.