Branfur studios does a very nice job painting his ODST helmets.
I know he uses fancy paint and a nice automotive sprayer, but I think to recreate this sort of look you could do the following:
Start with a base layer of a matte grey. Then, a darker grey or black for the area around the visor. After that, do a bit of weathering. I like to do a black wash, 50% paint 50% water, brushed on heavily and then dabbed off with a rag to leave paint behind in the crevasse and creases where dirt would naturally collect over time. If you want a cleaner look, closer to what Branfur used in those pictures above, use an airbrush and darken the notable geometry. After that, do some dry-brushing of silver or metallic paint to give the illusion of chipped paint. You could use other more involved methods to create authentic paint ships if you wish as well.
I think it's the combination of the matte paint, the airbrushed geometry, and the light paint ships that give Branfur studios' helmets that very clean yet well-used look.
After you've painted everything, seal it all in with a nice Matte or satin clear coat. Remember that the clear coat will ultimately determine the finish of your helmet. So even of you can't find a satin grey paint for the base coat, applying a clear matte coat over top a gloss coat will result in a matte finish.
