Why are barns red?

Hundreds of years ago, European farmers used a mixture of linseed oil, milk, and lime to paint the wood on their barns. The red on barns came from the addition of rust (ferrous oxide) to the paint mixture. When the mixture dried, it resulted in a dampened red color. Farmers decided to add rust to their mixture because it would prevent the spread of and kill fungi and mosses that would grow on their barn and decay the wood. Some people theorize that the red on barns came from including a slaughtered animal's blood in the paint mixture. When European farmers settled in America, they brought with them the soon-to-be tradition of the red barn. Although there are countless colors of paint to choose from at an affordable price today, farmers still have a proclivity to paint their barns red, perhaps for the sake of tradition or because it has become somewhat of a farming trademark.