Farmers originally used to seal their barns with a combination of linseed oil (red-ish) and iron oxide (rust, red). Then when paint came around, apparently red paint was the cheapest. bobvila.com/…/solved-why-are-barns-painted-red/
Solved! Why Are Barns Painted Red?
Any trip through the countryside might produce the same question: Why are barns red? Is it a matter of agricultural tradition or a product of necessity?