I've come across a few silver labs before. I have a dog that I always call a laborador mix. I say mix because we're not totally sure if her parents have some other breeds, but they both looked like pure black labs. They don't have any papers or anything, their owner was just very irresponsible and we ended up taking the pups to the vet and getting them their shots, and getting their mother spayed eventually. We adopted one of the puppies from that litter, and Chewy eneded up looking like a yellow lab, except she seems leaner and smaller than most yellow labs I've seen.I once came across someone who said they had a "silver lab"... it looked exactly like a Weim to me... and not just from colour, but this pup's entire body structure screamed Weim to me.
Anyway, in her litter there were 3 silver pups, 3 yellow pups, 3 black pups and one we called a rottie, because she had perfect rottweiler markings, but she grew up with a laborador body and a rottie head and coloring.