Silver labs are not really a breed...they are do to "cross breeding"
They don't come from cross breeding? It is just a dilute coat. They are as pure as any other labrador.

German shepherds are often diluted (blue) and they are purebred, and blue the diluted form of black in a doberman, and they are purebred as well. These dilutions don't result from cross breeding, they result from a gene from long ago suddenly showing up in a line, and showing up more as the line is constantly bred.