What a great answer! I appreciate the fact that you chose an utterly pointless question in order to present this splendid answer. Along with all the physics and chemistry that none of us readers would be thinking about if you hadn’t brought them up, though is something that is even more interesting to me — a correct or insightful answer to almost any ‘why’ question opens many doors. ‘Why’ is orders of magnitude bigger than ‘who’ or ‘when’ or ‘where’ or even ‘how’.

If I had undertaken to answer the barn question, it would have had nothing to do with astrophysics. It would have had more to do with economics, the fact that American farmers generally haven’t gone to the store to buy paint because they needed a lot of it and couldn’t afford the commercial product. They had home recipes for whitewash and home recipes for barn paint. And barn paint is what you get when you add bull’s blood to whitewash. The blood, besides providing a delightful deep red pigment, makes the whitewash resistant to weathering. Nobody wants to paint a barn any more often than strictly necessary — farmers have much more important things to do, like feeding their communities. They might have applied the same logic to their homes, except that barn paint doesn’t smell very nice before it dries.

You see? This is a completely different answer, but it supports your astrophysical answer. Barns are red for the same astrophysical reason as blood — iron is plentiful, and it has the chemical qualities that make it useful for binding oxygen, both in your blood and in your barn paint.

And I’ll bet there are several other answers, as well, just as different from yours as mine is, and just as illuminating.