Barns have traditionally been red because............

Barns have traditionally been red because it's typically the cheapest paint
color.
Barns tend to be red because of the high availability and low cost of red
paint. Paint is created by combining a colored pigment, whose hue depends
on the mineral that is in it, and a binder such as acrylic or oil to help
the color stick. Red paint is made from red ochre, a combination of iron
and oxygen. Iron is one of the most plentiful minerals, as the Earth's
crust is about 6% iron. Blue and green pigments are mainly made using
copper, which is much less plentiful than iron at less than 0.01% of the
Earth’s crust, making those paint colors more expensive.

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