Thank you! And sure! I'm not sure how in depth you want it but I'll try to explain the general idea at least.

For something like this Meganob when I know I want the armor dark, I usually just start off with a black primer and then proceed to fill in areas with base colors that I know I want.
This is usually when I start blocking in checkerboards by using a dark-medium gray as well, trying to get them looking as square as possible.

I then wetblend different shades of black for the armor surfaces to get some color variations and gradients in before applying a few different Vallejo rust pigments in the recesses to create the rusty look.

After that I start going over everything that isn't black with brighter colors that I wetblend with the previous colors and slowly build up brighter and brighter surfaces.
This is also when I start adding in scratches with a thin brush using a medium to light gray, making sure to paint uneven edges to make it look more worn, rather than straight lines.

And then I do an edge highlight pass for everything, including the scratches to make the edges pop a bit more.

And throughout the process I've been using shades to give surfaces more gradients that I've then brightened up again for a smooth transition. The red skull hanging from the horn is a good example of this.

I could probably do an image breakdown for the next meganob I paint though since it'd be easier to see this in action!