Why are some shotguns coloured blue and brown?

The receivers on most over-unders shotguns are given a plain silver finish but every now and again I see pictures of some with lots of blue and brown marks on the surface. What is this, and why are they not polished smooth like all the others?

Why are some shotguns coloured blue and brown?

The receivers on most over-unders guns are given a plain silver finish but every now and again I see pictures of some with lots of blue and brown marks on the surface. What is this, and why are they not polished smooth like all the others?