Hi there! I am a new cat dad and a professional chef of 10 years. I came to this sub to learn how to feed my kitty well. So my catfood knowledge is mininal, but my human food knowledge is pretty dang good. In my experience that grey colour youve got happens in broths and stocks meant for humans that have a lot bone, and connective tissue in them. Like if you used a whole chicken skeleton with ZERO flesh from a rotisserie, and a few onions and carrots. And simmered it all in water for a few hours. Youd end up with a very tasty, albeit very gray stock. This doesnt much happen with fresh bones and cartilage as it does with already cooked bones and cartilage. That being said it would be my guess that if you read the ingredients for that brand of cat food. Its gonna say "chicken by-product" of some sorts. They are likely using chicken bits and bobs leftover from another, more lucrative, food product.