1. Black
2. Yellow
3. Chocolate
There is a reason you rarely ever see a chocolate lab as a service dog. This information comes directly from some friends who work and train service dogs.
For years and years, conformation (show) breeders would not use Chocolate Labs in their breeding programs, because once you introduce the chocolate gene, you have a chance of having a Chocolate with the yellow gene, which gives you a "Dudley" - a yellow with a pink nose and eye rims. They are automatically disqualified from the show ring.
Thus the only Chocolates around were field types, which are bred for endurance, speed, and "style" (high-drive retrieving). They are way, way, WAY too hot for a service dog, but they are not stupid.
My 11 year old Choc is half field and half conformation, the product of an experimental breeding between a Black show champion and a Chocolate daughter of a Chocolate National Field Champion (NFC/AFC Storm's Riptide Star). She has a conformation style body but a field head (and a field brain inside it). She has her AX, AXJ, SHR, HR, JH, CGC and WX and she has already got two legs of her Rally Obedience title and started her CD Competition classes this evening. She is smart enough to open doors, step-on garbage cans, and baby gates.
Her full brother, also a Choc, has not scattered his forces in 3-4 different disciplines like we have -- he has his GHRCh and UH titles (that is as high as you can go in HRC, and is absolutely no picnic. To get his Grand Hunting Retriever Champion title he had to pass 6 rounds in the national hunting test competition). He's going back to the Grand in Vidalia this Thursday. Good luck Hooch!
My 4 year old Black is 100 percent field bred, very high drive, we think she's pretty smart but she is also ADHD so sometimes she acts like an idiot.
The Yellow is only 18 months old but is bidding fair to be almost as smart as the Choc.