Charles Leik, editor of The Barn Journal says that the question of “Why are barns red?” is their all-time most popular FAQ:
Ferric oxide (rust), a primary component of red paint, is inexpensive and this appealed to the thrifty farmers of New England and New York State. Red is the predominant barn color in that region. Natives of these areas were the early settlers of the Great Lakes states migrating there via the Erie Canal and the Lakes. I grew up in central Michigan and there were only a few non-red barns in our area. Two nearby farmers had gray buildings and soon one of them opted for white, which was also a rare color. I conclude that the early settlers brought their red barn tradition (and thriftiness) with them, and this was followed by the later immigrants (Germans in our area) who came directly from Europe.
How Stuff Works adds that “Rust was plentiful on farms and is a poison to many fungi, including mold and moss, which were known to grown on barns.”