Because mecha! I'm personally more into the modern organic looking mecha, I'm a sucker for Shirows spidertanks
punchy, noticeable attractive colors > camouflage when I'm painting. also, these are dwarfs, not pussy frou frou elves, they don't need no stinkin' camo.
First I mix elmer's glue with baking soda (you want a thick, but smooth paste consistency), and paint that thickly onto the base, then i dip that base in snow flock (train stores will have it). the snow flock will stick to the baking soda/glue paste, and the paste provides a thick cover for the snow flock and doesn't show the darker base color through the snow.
you can help prevent this by soaking your knives in lemon juice... okay, it won't really help anything except to make you very -very- cautious so you don't cut yourself XD
I would probably go over the metal with a light wash, it'll help give it more depth, though I suppose it could be a mithril ballista, it seems too shiny (if you like it as is, don't listen to me, using a wash, especially if you pick a light gray, will simply pick out the details a bit more and darken the cracks and seams, giving it more shadow, less a 'perfect' silver look ^.^.)