Barns are not red, but more of a rust color.
There’s a practical reason for this.
A barn is a big investment in both raw materials and labor, so early farmers looked for an inexpensive paint that would protect their wood from the elements.
They discovered that a good base could be made from skim milk, linseed oil, and lime. Then they tried various ways of adding color to the mix.
They wanted an intense dark color that would cover rough-hewn wood in one coat and not show every bit of dirt and grime.
Eventually they tried adding iron oxide extracted from rocks or rusty iron for color.
The resulting brick-red finish was cheap, good camouflage for stains, and quite durable.
Red barns have become such an archetypal image that many farmers still paint their barns that color, even when they’re using store-bought exterior paint and can have colors like Avocado, Frosted Lilac, Chartreuse, or Autumn Sun for no extra charge.