My chihuahua's skin is gray!?

I adopted my dog from a shelter when she was 5 months old. She's now almost a year old and the skin under her fur on her head and snout is starting to turn gray. There is no hair loss or itchiness or anything. I've heard this often occurs in larger breed dogs, but I don't know if I should be concerned. She weighs about 12 pounds so I don't know if she is a mix or not.