Barns Are Painted Red Because of the Physics of Dying Stars | Smart News
The upshot of this link? Because of nuclear physics, iron has the most stable nucleus of all the elements. It just does. All else follows from this: Because iron is the most stable nucleus available, when stars smash atoms together, they will make more and make stable atoms till they get to iron, and then they will stop. Therefore there's a lot of iron, therefore iron is cheap. It's red when it combines with oxygen, so it's the most cost-effective paint pigment around, and it was used on all the barns.
I'd turn it around and say that barns and Mars are red for the same reason. Mars is pigmented like a barn.
It's interesting to think about this ... even more interesting to think about what it leaves out from the equation. But it works for a nice little blog post.