Yes, I boiled red onion skins, reduced the it by boiling and got the resulting ink which I used to create this page of scribbles. In my experience, more onion is better. I boiled all of the water away, took the saucepan off the stove before it burned and added small amounts of water as needed to reconstitute the ink (otherwise it’s too weak to use). The color of the ink varies from a reddish purple to a gray.
Onion Skin Ink