These are the colors I used for metallics on both armies.
That Reaper Walnut Brown has quickly become one of my all time favorite colors. You may have heard you should never use pure white when painting models and I think the same can be said of pure black. This walnut brown is the darkest color I use and it gets worked into most of my models somewhere. It is a rich dark gray color, not warm and not cool. It is a nice neutral shade.
The P3 underbelly blue may not be strictly necessary, depending on the kind of look you're going for. I work this color into the final highlights to really get the metal to pop on the tabletop. Leaving it out probably gets a more realistic effect in the end, even if it's got less 'zazz' in the end. If you're just mucking about with this NMM for the first time, I'd recommend leaving it out. You can get perfectly fine results with the remaining trifecta.
The wood elves were my first attempt at NMM. These wardancers were the very first models I did and I will not lie, it took ages. Although I knew the blood effect would cover a lot of the blade, I wanted to practice the effect. (I also knew if I mucked anything up, I could just cover it with blood!)
There are two ways you can do the NMM in my book (The Book of Rodge). You can do the high level consideration of your light sources and the reflections between ground and sky and blagh, blagh, blagh. Or you can just highlight the metal like you would anything else.
The former works great on single models and seems to fall within the domain of competition painters. I have never seen a whole 2000 pt plus army done to this level online or in person. And understably so, that shit would take forever, son. (Although, I would be delighted to see one. Post a link if you know of any in the comments!)
The latter is very do-able. My close painting compatriot, the Ledge, is close to finishing a Bret army in that fashion.
My style falls between the two. On blades or any flat surface split into two halves, it's easy to do the "high" technique of flipping flopping the highlights on each side. On everything else, I use the "low" technique of just highlighting normally. That's pretty much all there is to it. You maximize the bang for your buck / wow factor on the blades and don't have to kill yourself figuring out and executing the proper highlight on a leg greave.
Here is an example of that on a wild rider. The rider's helmet was split into two halves, so it was easy to flip flop the highlights. The horse's helmet, although split into halves, is curved and split into small segments. I just highlighted it normally.
Other than the wardancers, wild riders and some characters, there wasn't too much metal on the wood elves, which is another reason I used NMM in the first place. You got to pickle your battles and deciding to go this route on an Empire cav army is a sure recipe for burnout.
The demons also used these colors, with some variations. First, I almost entirely abandoned the "high" flip flopping NMM technique. Most all the metals, even the swords, were just highlighted normally.
Secondly, after the the grays were done. I hit the whole model with Pearl spray as detailed in this post. The pearl spray lays down a layer of metallic pink, which shows up clearly in dark recesses but is somewhat hidden on bright highlights. For our purposese here, it just sort of un-NMMs the NMM!
Thirdly, dry metallic pigments were applied to the metal to further enhance the metallic look and give some subtle variation in the metallic coloring.
These Pearl Ex pigments are available from Jacquard Products. If you look closely at the swords below, you can see there are subtle differences between the blades (although generally pinkish overall due to the pearl spray). I applied dry with a brush and then spray matt varnished to seal them.
The demons are another army with not too much metal. The most so far has been this guy below, the "Rainbow Warrior" for my Treason of Tzeentch spell filler. I was kind of curious to see how the effect would look if a whole model was done up in it. With the amount of split surfaces, I did go back to the "high" flip flopping highlights technique. Although plenty of regular highlights are mixed in. You can see the color differences pretty well on this one.
That's pretty much it! I'm sure I'm glossing over something, so just post your queries in the comments and I will elaborate as necessary.
Not sure what's on my plate next. I've been mucking about with high elf metallics lately, trying to find a good look since that color is such a huge part of the army. I guess we'll see what I come up with!