There are two reason barns are painted red. 1.because farmers used to mix rust into the paint to keep fungus out of the wood. 2. they also would occasionally mix blood from a slaughter into the paint to try and conserve

Firetrucks

Originally barns were red because, hundreds of years ago, farmers would seal their barns with linseed oil, milk, lime and ferrous oxide, or rust, which killed fungi and mosses. It turned the mixture red.

The barns were used to cure tobacco, black absorbs/coverts sunlight to heat to improve the curing process

Pole barns are often stained, instead of painted. When they are painted it's often with white or green trim.