If you are looking at
NMM then bear in mind that it doesn't necessarily have to be clean.
NMM is all about creating a metallic effect using non-metallic paints, and while shiny is the 'default' way of doing it, it's perfectly possible to get some really grimy, worn looks from the effect as well. Just as metal in the real world can vary from shining chrome to rusty, mucky iron, so can attempts to replicate it in miniature form.
For example, this model is painted using
NMM, in that there are no metallic paints used, but it's almost the opposite of the standard clean
NMM:
The way to achieve this is to use lots of washes and drybrushes to really create the impression of the desired texture (look up some dark/worn/grimy metal in real life to use as a reference), but still think in
NMM terms (where is the light coming from, which areas are in shadow, what bits reflect what).
NMM is a method, not an end result, so you should be able to get your intended effect using it so long as you follow the theory. Bear in mind that by necessity, that picture you posted will be painted using
NMM, so if you really look closely at the colours used there you should get a good starting point to work from.
Hope that helps!