Actual answer: back in the day the sealant that farmers coated barns with often had iron oxide in it because it helps prevent rot and mold, and the iron oxide would turn the sealant mixture red. Now people just do it because it's a tradition.
It’s not mainly rust any more, they figured out a way to replicate the effect without using actual rust. It’s just pigment, and now red is probably cheaper because more people buy it because it’s traditional.
Blood is also red due to iron for the sane reasons rust is red. Rust isn't very vibrant on metal for other reasons, I'd assume mostly because it's mixed with something not clear.
I'm not sure if this is why, bit the color depends on how oxidized each atom of iron becomes, so if you have a mix of different oxidation levels, you would also have a mix of the colors
I thought red faded the fastest? It's always the first color to dissappear from advertisements in store windows. Also I remember hearing about people needing to get the red ink on their tattoos touched up after so many years though I think newer ink has improved