Hi, I have a 16 week old female yellow lab whose belly was bright pink at eight weeks and then around 12 weeks began to turn black. All my yellow labs had pink bellies as puppies and the skin turned white as adults so I have never seen this color pattern ...about 2/3 of her lower abdomen is black now with pink splotches! Is it because her mother was a black lab and two of her siblings were black so she got more black pigment and it’s manifesting as she grows? Her eye rims were pink when she was younger (although she’s not a Dudley) and now they’re darker so maybe that has something to do with it?
Yellows normally have black pigmentation that gives them the black nose, rims of eyes etc. It's often said that yellows are "black in a yellow jacket". Black skin isn't unusual at all in yellows, so I wouldn't worry about it.
Finn is a yellow lab with a black skin. He also has a black spot on his tongue. He is our third yellow lab, and the only one who has this. Our first lab was a yellow female and was the only yellow lab in the litter. Her mom was black and so was her dad. They both carried yellow genes. The first two had red noses, no dudleys...Finn is the first one who has a black nose. As snowbunny already wrote is not to worry. It is more common amongst the yellows!
Maisy was the same, her belly and some skin elsewhere (armpits) was black and looked dirty. Washing did not change anything so we thought it was just how she was, now though the pink has come back and she is 11 months, so things may change back for you yet.