Blood-Red Paint
Wealthy farmers added blood from a recent slaughter, and as the paint dried, it turned from a bright red to a darker, burnt red. Moreover, ferrous oxide or rust was often added. Rust was plentiful on farms and is a poison to many fungi, including mold and moss, which were known to grown on barns.
https://www.printmag.com/imprint/why-are-barns-red/