In the first photo Mojo is eight weeks and in the second and third he is two or older. This turning gray is not caused by the same gene as the puppies that are born gray nor is it the same as the gray that occurs with old age.
The other interesting thing about Mojo's coat is that each hair is banded with silver at the base, pewter gray in the middle and black on the tips.
Here is Judy Hartmann's Tru-Lee who started off fairly dark and now is, at one year of age, almost white or a very light cream.
Not all changes are from dark to light. Here is a very dramatic example of the ticking gene in Linda's Kallie. On the left is Kallie as a puppy and on the right as an adult.
Here is a dog whose coat is considered red which is caused by the presence of pheomelanin. The red dogs tend to be born darker, lighten as they lose puppy coat and then darken again as they mature.
Back to colour genetics.