It's a way of getting metallic colors with Contrast. Here's a droid I did with silver base and a coat of Aggaros Dunes (I think that's the paint I used, it's been a while). There are a few highlights with an actual gold paint, but those have been applied sparingly.
I use a coat of Skeleton Horde to tone down the metals on my Sisters. It's a lighter brown and results in a weathered or aged look more than a metallic look.
I haven't done all that much with this, but I'd say that whatever result you want, you should probably test the effect first before applying it on a large scale. I assume you're looking for a weathered look, given it's terrain? That seems to be a job for a lighter brown that can go thicker in recesses and corners but should be thinner or even diluted on surfaces and mopped up to prevent pooling in order to get a light tint only.
You can pre-shade with dark and light metal and put on the Contrast layer afterwards for a more subdued and integrated effect. Highlighting the traditional way, as I did on the Sister, is something I'd do if you're looking for weathering more than tinting, as it returns some of the untinted original color and gives you starker contrast and some clean surfaces.
I find that stippling and drybrushing work pretty well as a base over which the Contrast paint goes to produce a weathered look, which may benefit you on terrain, if that's what's you're going for.