HiI have it on what I consider good authority that at least some of them are purebred, as "pure" as any individual in a breed that has had a closed stud book only around 100 years. That is, from Dr. Mark Neff, a UCBerkeley researcher on the Canine Genome project, who has collected reports and pedigrees from breeders who have unexpectedly had "silvers" appear in litters. "You wouldn't believe" some of the bloodlines in which they are appearing; pedigree analysis shows the presence of the allele in the breed to go far back.
I trained a black Lab sired by a silver, and he was quite a good dog. I trained a silver bitch and she was difficult, but she was reared in a home with her dam, never separated, and that by itself could account for the poor training response, so I can't infer much about silvers from that. I did think she was ugly, not because she had the fineness typical of tightly-bred dogs, but because her structure was poor--narrow chest, turned-out front legs, stuff like that. She did have a typical Lab head fwiw.
I would certainly concur that breeding selectively for any one trait decreases one's ability to select for all other traits, and that breeding for two color recessives (b and d) necessarily means intense selection for color, thereby greatly reducing selection for anything else. I also take pedigrees with a dose of salt--so many ways for the parents of a puppy not to be who they are purported to be. DNA profiling, of course, helps with that.
Taken all together, I oppose the purposeful breeding of silvers because I think part of a breeder's responsibility is to maintain the essence or definition of the breed. Granted the Lab is a mix of older breeds, but it is a mix that was created with a purpose and image in mind. Reducing, not promoting, the variability inherent in the mix, and striving toward a standard, is part of defining, creating, and maintaining the breed. On the other hand, I don't get as excited as some here about it. We have a huge market for fad dogs providing motivation to the doodle and poo and silver breeders, and unless we can educate the public, trying to counter that motivation by force is a losing proposition IMHO. Pollution of the gene pool is not something I'm worried about. As long as they are crummy dogs, nobody trying to breed for working ability or show is going to cross to them, so they will stay in their own little corner. And thanks to DNA testing, anyone who wants to eliminate the allele from their breeding stock can easily do it in one generation.
Concerns about the gene pool are more in the opposite direction--loss of diversity, much more than presence of something unwanted. Unwanted stuff can always be bred out, but diversity, once lost, can never be regained once the stud book is closed.
I really believe the answer is DNA testing. That will determine if it's a lab or an outcrossing.
I had DNA test done on myself, very informative (I am not a lab)