I HAVE JUST STARTED WORKING AT A LOCAL VETS SURGERY (RECEPTION) AND WE ARE CONSTANTLY ASKED BY CHOCOLATE LABRADOR OWNERS THE IMPLICATIONS IN BREEDING WITH EITHER A BLACK OR YELLOW.  IE WHAT COLOUR WILL THE PUPS BE?  I DON'T UNDERSTAND, WOULDN'T THEY ALL BE CHOCOLATE?  PLEASE HELP, THE VET I'M WORKING FOR IS NOT VERY APPROACHABLE. 

This site might make it easier to understand - though it's quite a complex subject!

Basically it depends what other colours the parents are 'carrying' in their genes. Also, there are two 'types' of yellow; those with black pigment and those with liver, which goes to confuse the issue even more.

Of course there is a lot more to responsible breeding that trying to predict the colour of puppies :) if your role at the vets is to provide information and education to the clients please take the opportunity of including advise about all the other aspects.  To that end, a useful resource for you would be the Kennel Club their Accredited Breeder Scheme booklet lists all the recommended health screening for the individual breeds.  The Breed Standards lists contacts for all breed clubs at the bottom of the relevent breeds these will provide further guidance on health checks for individual breeds and codes of practice as regards such matters as minimum breeding ages.
Another useful resource for your clients would be a copy of this Post if you register you could PM Jane and ask if she minded you taking copies otherwise you could simply read, digest and pass the information on :)
It sounds like you could be in an ideal position to improve the knowledge and awareness of your clients to breed healthy, typical pedigree dogs thus improving the welfare of dogs and the happiness of their future owners, good luck!