Why are barns usually RED? (week 5)

This week we talk about color. It reminds me of a decade old question: why are barns usually red?

One possible theory is that farmers had lots of rust (ferrous oxide) laying around and mixed them with their paint. Ferrous oxide is poisonous to fungi and mold which often cling onto barns. Hence, the rusty paint was used to protect and preserve barns.

Another, albeit less likely, theory is that farmers had lots of animal blood laying around and used them to paint their barns. Hmmm, not sure about this one.