Remember this colour scheme from Rogue Trader?
These special release miniatures give me the opportunity to look at these kinds of chapters/ marines. You don't need to buy a whole army, just one model to experiment with. You may have noticed that this isn't a straight up grey - and you'd be right. Try adding desaturated blues (like games workshop's Incubi Darkness, or Formula P3's Coal Black) to add a bit of a kick to your blacks.
If you would like to learn how to paint this miniature why not sign up to the online tutorial service?
ONLINE MASTERCLASS TUITION