Hi, I'm a new chi owner. Adopted a 2 yr old a little over 3 months ago. Her belly was also pink when I got her and now the belly and chest are all turning a silvery-grayish color. From the previous comments, it seems this is a normal occurrence as they age, but it still makes me nervous. She acts fine..eating, playing, potty, and poops all normal. Guess I'm just looking for a little more reassurance. 
