The painted red strip around the outside of Roman Buildings

[Tarbicus]

This seems like a huge amount of a food product to waste by throwing it on a building.

Why? Cassein paint is made of milk product, so why not paint it on a wall mixed with blood? You're forgetting the more modern example of barns using exactly the same method, and there being no need to suppose milk would be less scarce then than in Roman times. Paint also helps protect the building itself, and it's far more expensive to have to rebuild than repaint.