Red barns aren't as ubiquitous here in western North Carolina as in other parts of the country. Unpainted barns were the norm for a long time. Still, that iconic red barn of children's books and country-themed calendars tugs at many of us. Our own red barn was unpainted for many years . . . and then John bought a spray painter.
I love the weathered gray of many old barns. But the way that red pops against a green background is nice. And the paint adds protection to the wood.
One mixture of skimmed milk, lime and red iron oxide created a rusty-colored mixture that became popular among farmers because it was cheap to make and lasted for years. Another preservative was linseed oil, derived from flax seed -- and flax, grown for linen, was a common crop. It was found that adding rust to the linseed oil created a nice rust-red finish to the wood.
What color are the barns where you live? If there are barns . . .