lookig at the pic i'd have to say go with airbrushing the armor. probably 4-7 layers including the primer.i'd do 2 layers of primer grey then white, a metallic base (color of your choice), then a flat version of that color, the metalic one again, and then one to 2 coats of clear coat
4-7 seven layers seeems kind of excessive, but I know that Nightmare would do that if it made the armor look better.
I believe that Nightmare used Automotive colormatched paint... I did the same when I made my armor. It did require A texture coat, then a primer coat, then a mettalic underbase, then the translucent mettalic green.
Also note that Nightmare airbrushes shadows into their details, and does a very heavy clear coat afterward.