We have been told darker and yellowish color can be a sign of a poor diet.
You have a staple (protein/calcium/vitamins) and veggie and fruit side with most diets. Some of your first gliders turned lighter gray and whiter on bottom the yellowing went away.
We believe this is because we feed them a good Ohpw and fruit/veggie diet.
A vet would also be a good place to go if your really concerned. A wellness visit for us is about $90