My cousin would like to know about the permissibility of making ink on Pesach. The ink recipe includes flour, fruit juice, vinegar, and a little water.

According to my cousin, it's not ראוי לאכילת אדם (edible for humans), but it might be ראוי לאכילת כלב (fit for canine consumption); while it tastes too bad for humans to eat, dogs would probably be okay eating it.