Happy Birthday Murphy!!
Marty, Jorja's coat has a common genetic trait that dilutes the coloring...Chase has a few white hairs too, but not many. I have seen other chocolate colored dogs with an almost greying to their coat.
It is much like the dogs that we own when they turn silver.
I think that the coloring comes from the Poodle genes. It could get more pronounced...or it might not...hard to say since neither parent have the pattern in their coat...but it could be a throwback from somewhere else in the genetic background.
Here is a link that explains the dilute gene:
http://www.canine-genetics.com/pdlcolor.html