Why Labrador Retrievers are Different Colours.
I's all in their genes!
They have a gene that produces a protein, called Eumelanin, and another gene that produces a protein that properly synthesizes eumelatin into a black pigment and densely packs this protein into the fur coat (hair follicle). The Gene for Eumelanin, when mutated or non-dominant, produces phaeomelanin instead of eumelanin, which is because the receptor MC1R is unable to synthesize the eumelanin. The second Gene locus is important for TYRP1 enzyme, that stabilizes intermediates in the melanin synthesis pathway.
Gene Locus E = Eumelanin (makes the coat dark)
Gene Locus B = TYRP1 (Tyrosinase-Related Protein 1)
So a ressive a gene but dominant B gives the yellow colour with black pigmentation (nose for example), whereas if the dog has both the reccesive E and B gene, you still get a yellow coat but a pink nose.
The black labs are dominant for both genes, and the chocolate labs are recessive B (bb) so you get some eumelanin, but because the synthesis isn't complete/correct, you get brown fur.