Barns were traditionally painted red due to the accessibility and cheapness of red paint. Farmers could create red paint out of skimmed milk, lime, red iron oxide and linseed oil to protect the wood of the barn. The linseed oil and red iron oxide gave the paint its red color.
To further deepen the red color, wealthy farmers sometimes added blood from a recent slaughter or rust to protect the wood. Rust was readily available on most farms, and it naturally protected wood against fungi like mold and moss. Eventually, whitewash became cheaper to use, and it replaced homemade red paints in popularity.