Before the 1700s, farmers did not paint their barns, they allowed the weather and nature to treat the buildings. This began to change by the late 1700s when farmers, especially in Virginia, painted their barns a colonial gray. Around a decade later, red became the go-to color for barns.
The main material derivative for red paint was rust, a compound that was easily obtained and cheap to produce. When you combine rust, milk, lime and linseed oil, the resulting paint had a red-ish hue. Thus traditional “barn red” became a popular and cheap paint that is still widely used on farms today.
Bob Crittendon, Barn in the U.S.A., Golden, Colorado: Fulcrum Publishing, 2006, 87.