Thanks everyone !
I should have specified that NMM stands for Non Metallic Metal and, as mentionned by Atheling, it consists in recreating the lights and shadows on metal parts without using any metallic pigments. Some people do it on a black undercoat but I went for a white one (zenithal to be precise, as this way it remains dark in the recesses), starting with glazes of greyish blue and greys then a Nuln Oil wash mixed with a bit of Drakenhof Nightshade wash, finishing with light grey and pure white highlights. As for the gold, it is a Vallejo greeny yellow as a base followed by a Gryphonne Sepia wash then highlighted with off white and pure white. The trickiest part in this is to actually put the highlights in the right places, where light is supposed to be reflected, needs a bit of practice to get it right.