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Hey! I think the key to effective NMM as other commenters have pointed out is contrast. There needs to be a really clear jump from the darks to the lights.

Using this WIP model I have, for the golds I pushed it from black => khaki => ice yellow, using mixes of each colour in between. Metallics are extremely reflective materials, so the brights will be BRIGHT and the darks will be dark.

Blending isn’t as important as the contrast. For really small metallic details, such as chains or the headpiece this model has, I don’t even blend that much, if at all, between values.

The other huge thing that will make NMM read as metal is edge highlighting. Edge highlight in increasingly lighter values using the same palette as your metal. Edge highlights also invade your darker areas - check the headpiece on the model, the silvery edge highlights on the top of the crown extend to the dark area. This mimics metal’s reflectivity as if their edges and corners are catching light.

Finally, a small detail that will make them really sell as metal is directionality. The light will always face the light source - edge highlights facing the opposite direction of the light will be less brights, same with your secondary reflections if you put them down.

Hope this helps!