What the heck is NMM? Non-Metallic Metals. Still confused? Basically, it's where you use regular painting colors (blue, white, gray, etc.) to create a dynamic effect that makes the model appear to have been painted using reflective metallic paints. But because you're actually applying the mirror effect, you can make the surface as colorful and dynamic as you'd like, and always control where the reflection is coming from on the model despite any actual source lighting in the room. Here is a great example from Blackmoor at www.blackmoor.ca:
While the model appears to be painted in bright chrome, it is actually a series of layered colors to make it appear to be a highly reflective surface -- much more reflective and dynamic than you'll actually be able to achieve using an actual metallic paint on such a small surface.
While I've never tried my hand at this personally, Blackmoor has a great article (http://www.blackmoor.ca/articlesnmm.htm) that covers different color combinations for different kinds of reflective surfaces. While NMM painting was all the rage just a few years ago, the technique appears to be falling out of favor among miniature painters... probably because it is extremely complicated to do, and would be mind numbingly crazy to try to apply to and entire army in detail. Regardless, it is hard to argue with the results. I'm going to have to try this out sometime... Until then, enjoy the fruits and labors of someone else's work.