Here's the 'Eavy Metal version
The most important thing you can do for a quick paint job involving power armor is edge highlighting. The little lines of gray at the edges of each piece of armor go a long way towards creating some depth in an all black model.
And although this photo doesn't show it, you should paint the non =I= shoulder pad in the chapter color and insignia of that particular marine. Fluffwise, this is to honor the machine spirit of the armor, who doesn't take kindly to the coat of black applied over the chapter color(s).