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.
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