Here's a way to make nmm using gw paints.
ScorchedBrown, SnakebiteLeather, DesertYellow, BleachedBone, SkullWhite.
1. Start with a base coat of scorched brown.
2. Cover around 80-90% of coat with snakebite leather, the remaining 10-20% being the darkest areas.
3. Paint thin layers (very thin) of desert yellow on top of the snakebite leather.
4. Mix Desert Yellow with Bleached Bone, then highlight the areas you want to be reflecting light.(really think where to put your highlight, it's the key to nmm)
5.Keep adding bleached bone to your mix as you paint multiple thin layers of highlight. Keep doing this until desired effect is achieved or your mix becomes pure Bleached Bone.
6.At this point you mix Bleached Bone with Skull White and continue your highlight.
7.For your final highlight just take your finest brush and paint little pure skull white dots where you want your most striking reflections.
Now this is 1 way to make nmm. It's not the best but it's an easy linear way to do it. There's no wash, shading... involved, only highlighting from your darkest color to skullwhite.
You can use the same technique to make almost any color of armor/metal, just find the right colors and keep highlighting until you reach pure skullwhite.