Many breeders of silver labs will quote the stance of the AKC on silver labs as a reason for why they are legitimate. However, the AKC is simply a registrar and do not control what is accepted as the breed standard. This authority falls to each breed's parent club. The Labrador Retriever Club determines the standard for the breed, not the AKC. For this reason, silver labs are not accepted as purebred labs and cannot be registered truthfully through the AKC. This is why many breeders will tell their customers to write "yellow" when registering a champagne "lab". 

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For more information, please see the Labrador Retriever Clubs official stance on silver labs.

https://thelabradorclub.com/wp-content/uploads/2018/05/The-Issue-of-the-Silver-Labrador-Fran-Smith.pdf