There are a couple of things you need to keep in mind when executing
NMM.
First and foremost, lighting. As it has already been mentioned, you need to establish and keep your lighting consistent. Whether it's one source, two, or many more, as a painter, you need to figure out where you want a model to be lit from, and how, and keep it consistent throughout the model. I recommend starting with one strong light source, and then as you become more practiced with
NMM, you can experiment with multiple.
The second is contrast. I'd recommend doing a quick Google search on "knight armor" for a point of reference. Take a look at the lights and shadows, and notice just how extreme it moves in values from light to dark. The brightest points are almost invariable white, and the darkest points reach not always black, but an extreme dark, depending on the environment. One of the tricks to selling
NMM is the contrast. Putting a bright highlight next to a dark shadow really helps communicate the "texture" (that is, the reflectiveness) of the armor. The degree of transition will also really depend on how reflective you want the armor. Compare something that is quite matte (like brushed metal) versus a highly reflective metal like chrome. You will see that a matte metal tends to have softer transitions, while chrome is very hard, with very defined jumps between light and dark.
Third is reflective light. Obviously, metal reflects. And not just light, but what's around it. Metal objects will capture and reflect light and color bouncing off of materials around it, and you can capture that with secondary highlights and playing around with color values so that a silver metal isn't necessarily just "grey".
Consider this cylinder.
We have a single light source shining light directly onto our object. The light will reflect strongest on planes that directly face the light source, and then transition to shadows as the planes angle away from the light. We must also consider the environment (sky, ground, and external objects) and the light that they reflect from the light source onto the object. It should be noted that these reflective lights act as light sources themselves, but unless they also emit light (or are 100% reflective), the light they cast onto the object will never be as strong as the light source itself.
When we add color into it:
If our light source is the sun (which emits a yellow light), our highlights as a result of that will have a yellow tone to them. The reflective light from the sky is blue, and from the grass is green. Our object, which is purple/pink, will reflect a purple/pink tone.
So you can see how a grey metal can be all manner of color based on the light that is reflecting from the environment and objects around it. The closer the source of the light is, and the degree of reflectiveness of the object (and objects casting it) will determine the strength of that color.
Lastly, consider your model as an example. I've done a quick Photoshop pass on it to show how you could strengthen some of your lights and shadows, and add/removing lights and shadows to better showcase the
NMM effect. I've indicated the primary source of light.
Also note that the closer the object is to the light source, the stronger the highlight. The helmet and collar are brighter than the gun barrel, which in turn is brighter than the foot.
Hopefully that helps. Keep experimenting, and when in doubt, Google for references!
