There is scientifically no possible way your dog is a purebred pug. It's not an opinion, or a fact that he's purebred pug, it's just misinformation.
You wouldn't look at a big cat that's tan with a furry mane and say "that's a tiger." You wouldn't look at a labrador with long, flowing hair and say "it's a long-coated labrador". Certain things, like size, certain colors, coat type, etc. are subject to "throwbacks" or mutations making them show up in breeds where they don't usually show up, but the traits your dog have are not those traits, and anyone with any kind of education in genetics could tell that dog is not a purebred pug.
Why are you so against acknowledging that the dog's parents must have some either boston terrier or french bulldog in them to create that color (and more than likely a healthier dog than most purebred pugs are to begin with)?