The fine particles in the average dust storm on Mars will definitely stick to many structures due to static electricity, so that's another possible cause for the 'rusted' lookCrimsonSky said:The rusty particles on the exterior of my Mars City structures was blown there by dust storms. That's how i see it.
True, there are more sources of oxygen aside from the atmosphere, but the effect of the latter will usually be far more significant.If you want to get really technical, rust can form not only from oxygen in the atmosphere, but by chemical reactions from minerals in Martian soil.