I saw this trick while watching some other painters on You tube and thought it would be perfect for these Orcs. it is very simple to do and the results are great.
When the armour has been dry brushed and washed with a red brown, simply dab on some 'metal' coloured paint with a sponge. Like dry brushing make sure the sponge is not over loaded with paint and the effect is great for rusting metal. I might try the same technique for adding mud etc.
I had never tried this before and was pleased with the results, you're never too old to learn new tricks and watching You Tube for tips has become a pass time for me.