Is it safe to use steel shot in my 10-bore shotguns?

Is it safe to use steel shot in my 10-bore shotguns?

Steel shot: I have two 10-bores. One is a Gunmark Kestrel with 3.5in chambers rated at 1,200kg/cm2. The right bore is machined to 0.751in and the left bore 0.729in. The second gun is a Zabala LP71, which also has 3.5in chambers rated at 1,000kg/cm2.

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?