So, why are barns painted red?

"The Archives of the Shiawasee County Historical Society explain that a week or two after a farmer would put iron scraps into a large barrel of buttermilk, that buttermilk would be transformed into a durable red paint. That this paint was so easy and so inexpensive to make explains why so many barns were red in color."

Source: Michigan's Heritage Barns - Mary Keithan - Michigan State University Press.