I did own a pug, a black pug, and, with the exception of puppyhood he never bit or attempted to bite anyone. (In puppyhood, it was playbiting, and I don't recall much about it....just know that all puppies playbite, so I'm sure he did). He was extremely mellow and good-natured. Based only on him, I would say they are the most laid-back dogs in the world.
However, a good friend of mine had a fawn pug, and she could not be trusted with children. She was a little nervous around them and could not be trusted not to bite. Of course, when she was younger, her own family's children played very roughly with her, so I don't know if that probably had something to do with it.
The differences probably were in how they were treated when young? I've wondered if the genes that carry color also carry some temperament traits. (I've never met a red/orange cat that wasn't a complete spaz...meant in a good way, I have an orange cat!!)