
Hi,
Your lab may have a throwback with regard to the colour. The oldest breeds of retriever are the Curlycoat and the Flatcoated retriever, these were developed from the St. Johns Labrador, which was a brindle coloured boat dog, the modern labrador also came from this common ancestor. As each breed was developed the original breeders changed the type and colour and working ability through crossing with other breeds. Even now there are some golden retrievers which will have black hairs, and in flatcoats it is not uncommon for dogs to have white hairs. All the retriever breeds were interbred up until the late 1970's, and dogs from these litters registered as which type (or breed if you like), they resembled. Hence some labradors will have a definate wave in their coats, along the back, which they got from the flatcoat.
During the early years of the flatcoat it was not uncommon for dogs to have ginger tan eye brows or brindle legs, there are some born right up to the present day which have white flashes of hair on their chests!
From what you are saying, I do wonder though whether your puppy has a lot of chocolate breeding, which has come out as a patchy area on the coat. To keep a chocolate line dark in colour, you need to breed to black dogs carrying chocolate colouring. To breed too often chocolate to chocolate or to breed chocolate to yellow, you do get very wishy washy colours. I had a flatcoat which was from a black sire, and black dam, both carried liver colour, (which is the same as chocolate), very strongly. When the puppy changed coat, it ended up as an adult with a black back and liver every where else! Equally, your new puppy may change colour again one it has shed it's puppy coat. A friend had a flatcoat which was born a grey and it turned jet black after shedding it's puppy coat.
The mature coat colours for labs are as John says black, yellow, and chocolate. Flatcoats have two recognised colours, black or liver only, however the dog and bitch, when they mate do not let this worry them, and will happily produce yellow pups for us! The yellow in flatcoats is a throwback from it's earliest beginings in the 1800's!
Have you spoken to the breeder about this? They will (or should) know the line they are producing puppies from, better than anyone else, they may have seen this before.