The funny thing is, if you go back and look at the time he's referring to, most barns were white (painted with whitewash), not red. It was much easier to make whitewash; only people with access to lots of iron oxides can make red paint. Red paint being the most common color for barns is a relatively recent phenomenon.
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.
pg fell for the just-so story. His ideas, while interesting, are not supported by the literature of the time. Whitewash is absurdly cheap, and it's typically based on lime, which is pretty much the first thing any developing society needs to obtain in large amounts.
https://news.ycombinator.com/item?id=5822301
https://news.ycombinator.com/item?id=9045513