That is what happened to my 2 year old Yorki/Poo. His tummy changed from a puppy pink to more of a tanish color. In some places, like under his fore-arms, it is a grayish coloring. When we went to the vet, he said it was nothing to worry about as it happens as dogs get older and leave puppy hood. Lots of our dog friends have had this happen, too as they reach a year old.
"If your dog doesn't like someone you probably shouldn't either ~ Unknown
MommaMia, Thank you. I was worried about her. Since she's a cream/apricot color, with the skin darkening, it looks like she's dirty underneath (kind of like a blonde who doesn't have her dark roots touched up). I guess I'll have to let her hair grow longer so the skin is covered with lots of hair to cover the darkness.