Originally the red color was actually a by-product of additives used in the paint mixture. By adding ferrous oxide (rust) to the paint which is poisonous to mold and moss, the paint would act as a deterrent to these. Ferrous Oxide was inexpensive to buy and lasted along time as a protective paint for the wood. Also it is sort of a tradition across the south but the custom actually originated in Europe.
I always wondered why barns were red. Fascinating. They are wonderful structures to see and something I could never tire of. This is a great photo of a barn. I love the composition!