We have a nice little red barn near our house, and whenever I see it and all the other red barns in the country it made me wonder, why is red a favorite color for barns? You see it used a lot in Sweden, and all over North America too.
Originally barns weren't painted at all but to extend the life of the wood farmers needed to invent a way to protect it from moisture and mold. An inexpensive recipe they invented included linseed oil, milk and lime. Adding ferrous oxide (rust) to the mixture helped prevent mold and mosses growing on the wood. Ferrous oxide is so easily available that it adds almost nothing to the cost of making homemade barn paint. That addition to the paint did a good job of protecting the wood and made it look a rusty red-orange color. This was the beginning of the red barn tradition.
Once paints were mass-produced farmers continued to choose red, and not just to honor the tradition of their homemade paint color. Red is the most inexpensive paint color to produce. Since barns are huge they require a lot of paint so the most affordable red paint became a logical choice for painting barns.