Here's a bit from a labrador breeder who is a friend of mine: (taken from her website)
White Spots
To analyze the reason why some black Labs have only a few, not-easily-seen, white hairs on their chests while others have small white spots, it is best to first picture that all Labs are white--the condition of having no melanin production. The gene loci for color control both the color of the pigment as well as the distribution of melanocytes throughout the body of the Lab. Therefore, in a black Lab, although color is determined by alleles at the B locus, alleles at the A and E loci determine even distribution of the color over the entire surface of the coat. Labs that carry an allele other than "As" at the A locus, have a greater likelihood of expressing more white hairs than those Labs that do carry "As". Therefore, although all Labs should be homozygous for the S allele at the S gene locus, some may still express white hairs on the chest, bottom of the feet and under the arms and groin areas.
If anyone is curious, check out her link here for other examples of mismatched:
http/www.woodhavenlabs.com/mismarks.html
As for silver, here's a bit
Silver Labs
The silver coat color in Labradors has gained much attention recently and is a very controversial topic (see The Labrador Coat Color Controversy: Do Silver Labs Really Exist?). Reasons for the controversy stem from the lack of information available to trace the origins of this color in the breed as well as the fact that the AKC standard for the Labrador breed does not acknowledge silver as an acceptable color for a Lab. Some breed enthusiasts consider the silver coloration to be a sign of impurity of the bloodline
and a link on "silver labs"
http/www.woodhavenlabs.com/silverlabs.html
Hope that helps some.