This reply may be a bit late but if someone's interested here's how I painted mine:
I'd start with the armour since it is the messiest part. A fast AND good looking method to paint those (especially the Morannon Orcs with their heavily structured armour) is to give them a really rusty look by undercoting them with a middle brown tone and roughly stipple different kinds of orange over it. I did a light drybrush with a dark metal colour before shading with a mix of black and brown shades. Afterwards put a highlight with some bright silver on all sharp edges.
Orcs are not that interested in keeping their stuff clean anyway
Change the water for your brushes to get rid of metal particles Swimming in it.
For the clothes I'd go with a small palet of colours that are mixed with each other in different proportions to make your army look coherent but still give the impression of each orc having different stuff. Same rule goes for skin.
I hope you found this helpful