I'm with Fleo_Werbroth, NMM is not well done. The brush technic is really good on the whole miniature, but the NMM does not give the effect, it just seems that the armor is painted blank and yellow. But the rest is well done as I said, even the NMM looks well painted (not giving the effect but feels cool). I can't give you any other advice rather than observartion of real source material for the correct emplacement of shadows and lights. But as I said, no real advice, not a huge fan of NMM (looks a bit cartoonish for me) so in my case I tried to improve my metal color technic. Sorry for the long paragraph!
Keep on rocking! Saludos