In the late 1700s, New England farmers, in the northeastern US, began coating barns to protect the wood.
The coating usually mixed linseed oil, a plant oil, with minerals such as iron oxide, the red-brown compound in rust, and turned that same rusty red-brown.
When ready-made paint came along, many people kept the red tradition. It helped that red paint is among the cheapest, because iron oxide is plentiful in the Earth.