Well, demons can throw fireballs...
Also, I'm thinking ein might be on to something.... dull grey DOES make goblins look and seem a bit too hardcore. I'd like to see what a greenish-brown goblin looks like.
Green looks too cartoonish. Light grey looks too much like zombie-dead type nonsense.
My personal view of goblins... is that they are highly militaristic, organized and evil and they all wear military uniforms with esoteric and largely unknown little decorations pinned on the outside in no random order, as a point of pride in their society. Not so much the rag-tag look as like typically portrayed. They take trophies and make leather earrings out of humans and rings out of the femur bones of dwarfs, so a uniform covered in medals made of the folks they killed could be interesting!
I'd like to see Fault's avatar with greenish-brown skin and see what it looks like.
Alternatively, we could also think up of the different types of Goblins and make their own backgrounds.
Goblins, Hobgoblins, Maglubiyet, Bargrivyek, Khurgorbaeyag, ect...
Also, yeah, I see humans as dirty, smelly, leather-wearing europeans with sewage caked on their boots and all that. Stereotypical fantasy-era humans, although the more esoteric, balaclava-wearing middle-eastern sorts with scimitars and camels makes sense to me as well.