Ever wondered why barns are red?

Years ago, many farmers would seal their barns with linseed oil, which is an orange-colored oil derived from the seeds of the flax plant.
To this oil, they would add a variety of things, most often milk and lime, but also ferrous oxide, or rust. Rust was plentiful on farms because it
killed fungi and mosses that might grow on barns, and it was very effective as a sealant. It turned the mixture red in color.

When paint became more available, many people chose red paint for their barns in honor of tradition.
(Farmers' Almanac)