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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