The American Kennel Club (AKC) not only registers purebred dogs, but also mixed-breeds. In some cases they have registered mixed-breeds as purebred dogs, and they still do.
To be sure that you acquire a purebred Labrador, which would enable you to take part in every official Labrador competition, you should check the website of the AKC-parent club of the Labrador Retriever, the Labrador Retriever Club, Inc.
This is what the LRC has to say about so-called “silver”, “champagne” and “charcoal” “Labradors”:
BUYER BEWARE! TRUE LABRADOR RETRIEVERS ARE BLACK, YELLOW, AND CHOCOLATE ONLY!
Further more, since not all “Labradors” sold by dilute breeders actually are affected by the dilute gene, but do carry the dilute gene (these dogs are called “silver-factored”, “charcoal-factored” or “champagne-factored” and will pass the gene on to their offspring), we advise you to demand a dilute test of both parents. The only acceptable test-result is D/D (clear).