NMM stands for Non Metallic Metal. It's a technique that has become something of a staple in fantasy miniature painting of late, that consists, basically, on painting the reflections of light over metallic surfaces without actually using metallic paints. Like greys and blues for steel, or browns for gold/brass.
It is used mostly on rather smallish scales, like 28 to 35 mm models, since these sizes benefit more from the exagerated contrast between highlight and shadow than a regular 54 mm model, although there are some masterfully rendered examples of the technique used on such large models.