Hi, and welcome on the CMON-forum.
NMM is an abbreviation for Non-Metallic Metal. This is a technique now incorporated into miniature-painting, which was allready used in 2D-art. It mainly comes down to painting surfaces you want to look metallic without using metallic paints (paints which contain tiny metal particles). So you would use a blueish grey to simulate silver, a brownish yellow to simulate gold, a redish brown for copper, etc. The surface should be painted in such a way that it seems like it is reflecting it\'s surroundings (and light) as if it were a real metal surface. This is something metal paints normally do for you; with NMM you have to do it all by yourself.
One \'problem\' of NMM is that it photographs very well, but sometimes looks a bit weird when looked at \'in person\'. This is due to the fixed reflections, which look good under certain angles, but slightly less good under different angles.
There are some good tutorials on NMM in the articles section of CMON.
Bye,
Jacky Visser
ps. *doh*, seems like Mosch beat me to it.