Yellow is produced by the presence
of a recessive epistatic gene which has the effect of
masking the the black or chocolate genes.
"The Labrador Retriever coat colors are black, yellow and chocolate. Any other color or a combination of colors is a disqualification. A small white spot on the chest is permissible, but not desirable."
The above quote is from the AKC Labrador Retriever Standard. There are only three recognized colors in Labradors. A white spot on the chest is permissible. NO other colors are recognized. However, other colors can, and do, occur. Brindling, black and tan, dilution of the chocolate gene which gives a chocolate a gray (or silver) appearance. These colors are a disqualifying fault, and are incorrect for the breed.
Do not be fooled into buying a puppy in any breed that the breeder claims has a "rare" color for that breed. A reputable breeder will tell you if a puppy has a mismark or incorrect coloration and will require you to spay or neuter that dog to prevent the reoccurance of the disqualifying fault.
Mismarked or incorrect coloration does not affect the ability for the affected dog to be be a good pet. A labrador is a labrador regardless of its coat color.
These pages are a guide for determining what coat colors you can expect from a given breeding.