Paint layering for Armour

Silver paint tends to have spotty/thin coverage in my experience, so, from that standpoint, a base coat of gray makes a lot of sense. I usually just paint the silver/gold/bronze or whatever straight onto the figure's armor, apply touch-ups on the thin spots, if necessary, and then apply a black wash over that to dirty it up a bit and bring out details. Alternatively, a black (or similarly contrasting dark color) base coat is a good idea if you're dry-brushing the metallic color on top instead of applying a complete topcoat.

Another benefit of multiple layers of paint on armor is that you can scratch it up slightly for realistic-looking battle damage (glimpses of the undercoat are revealed by the scratches/dings). For example, let's say you were painting Boba Fett's helmet, you could give it a base coat of silver, paint the green on top of that, and then strategically gently scratch/sand the green to reveal the silver, making battle damage/wear.

Thanks for the answer. I don't do any primer. Just never saw the point imo. So in your opinion, for metal armour, black coat with dark grey on top and then an over coat of shiny silver will come out nice? Or is it the old "try on a scrap figure to see how it turns out" idea.

That grey may have been a primer coat, which many custimizers put down as a base coat because it covers well and it helps the later coats adhere to the plastic