Here I tried adding alum to a small amount of pulp and vinegar to another. Neither changed the colour one bit! Still it wasn't such a bad idea, it might have worked.
Although I added it cold - when I added the red onion skins the pulp was in a saucepan being boiled, I later learned that you don't need to boil the pulp anyway.
Someone suggested adding cut up red table napkin to the pulp, paper ones of course - I think I'll try that with the next lot.