Yellow shell may be malnourished and healthy. The turtle shell is green. You should buy some shrimp or other food. There is nothing wrong with eating it. If you eat meat, I recommend eating shrimp to help the turtle shell harden. You can't change it directly with tap water, and you may die. Tap water should be left on the balcony to dry for 2-3 days before changing water.
Ask questions if you have any questions.