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Farmers originally used to seal their barns with a combination of linseed oil (red-ish) and iron oxide (rust, red). Then when paint came around, apparently red paint was the cheapest. https://www.bobvila.com/articles/solved-why-are-barns-painted-red/
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Basically also why Swedish barns are red. I presume those two stories and red barn origins are related.
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Barns are red because supernovas produce significant amounts of iron.
https://futurism.com/how-red-barns-are-linked-to-dying-stars
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