I'd say you could use pretty much any colour you want, mainly greens and browns, maybe even darker flesh colours. All orcs have different skin colours, so use your imagination and be creative
Once you've picked your skin colour, I'd give the fig a basecoat, then gradually mid some Kommando Khaki into the base colour for highlighting. Afterwards, give the skin (and the rest of the fig for that matter) a generous wash with a mixture of inks. For dirt, I usually wash with Badab Black, Devlan Mud and occasionally some Gryphonne Sepia, too. Make sure each wash is dry before you do the next though, and make sure it isn't too thick to mask any detailling
That's my take on it anyway; I've only painted a handfull of orcs and goblins in my time but I think it gives a good result