Why barns are painted red (answer: physics of fusion)

Yes, but if you're like me and went back and read all the old journals and almanacs, you'll find that almost nobody made paint this way. Paint was made from flax seed / linseed oil, lime and milk (all of these are more plentiful than animal blood), not iron oxide or blood. Other alternatives include eggs (also more common than blood), flour and dirt.

(source: hours of reading up 100+ year old farmer's books, since I work with Yonatan, and based my replies to him on primary data. In short, he took a very nice science concept and wrapped a just-so story around it).

BTW, if you're interested in recreating society after the fall, various calcium/carbon products like lime and chalk is going to be the first thing you need to obtain after a good supply of water and food.