Example of dramatic color change:
As mentioned, coat color changes are expected. However, sometimes, the change can be more than you could have imagined. Have a look at little Dudley (below). The first photo was from May of 2015. The second was more recent, after a ring of pure white hair grew in to frame his face.
This amazing color change is due to the graying gene (G). Despite the name, the graying gene can cause hairs to turn white. Quite a few breeds can have this gene, including the Pomeranian. This gene causes graying as a dog matures (the dog is not born with the lighter color); it can begin to happen at any time, at any rate of speed, and is not related to graying that occurs with some dogs in old age.
Essentially, the graying gene causes some hairs to not hold color as they otherwise would.
And the reason that the white appears as a circle is that this Pom genetically has a black mask, which keeps the face black.