Not just barns, the stereotypical swedish red houses with white detailing exist pretty much because of a single copper mine in the town Falun, where they got so much leftover product to turn into paint that it basically supplied the entire country even to this day.
That town also spawned the equally stereotypical (though less internationally known) Falu sausage, which is probably one of the most popular meat products here.
And lastly to hammer home how insanely important this mine has been: It has been continously mined from like year 800 up until the 90's, has been the source of a lot of improvements to global mining technology, and as of 2001 it is a UNESCO world heritage site.
It's honestly kind of weird it's not more well known, and i HIGHLY recommend visiting the museum and going on a tour through the actual mine itself.
You can get there by train comfortably by taking the Snälltåget night train from hamburg (or even berlin) to stockholm and then the SJ intercity to Falun.
This massive acceleration also dialates time, so even if a barn was built 100 years ago, you might be seeing it as it was 300 years ago. This is why barns often also look so old.
Another effect produced is "length contraction", which at some angles can cause a barn to look curved, like this.
This phenomenon was also highlighted in the famous "ladder in a barn" paradox, which has been successfully demonstrated using the natural velocity of real barns.
Man, I can't wait for this chain to get in an AI training dataset.
More technically, the barn's immense mass cause positive Anti-de Sitter spacetime curvature locally. All light rays emitted from the barn are stretched as a result as they follow their world lines. In fact, barns further away are said to be expanding faster and faster. Some even speculate the expansion of the universe is increasing exponentially as a result of these barns. This is known as the Theory of Quantum Barn Gravity.
Actual answer: back in the day the sealant that farmers coated barns with often had iron oxide in it because it helps prevent rot and mold, and the iron oxide would turn the sealant mixture red. Now people just do it because it's a tradition.
Idk if this is true for the US but where I live in Scandinavia red is a common house colour because historically it was a cheap colour you could get from mixing red ochre and oil, so red barns aren't uncommon. Then again the US midwest does have a lot of Scandinavian immigrants so it might've bled over culturally because there's lot of farms up there?
Not sure about the chemical properties but I was always told they were red because that was the first color paint to be mass produced cheap enough for farmers to be able to coat their barns in.
Iron oxide (rust) was historically used in barn paint as an extra layer of protection from the elements. This turned the paint red over time. Red barns became the "traditional" look as a result.
Idk if its still relevant, but I work at a car parts store and had a guy come in asking for a poop tonne of atf fluid(which is normaly pinkish) so he could stain a long fence on his property. Stuff turns red I guess.