When should I be changing my bottles? When it turns yellow or?

I would check the bottles and the bottle nipples at least every 2 to 3 months for signs of wear and tear. Breast milk or formula should drip steadily out of the nipple, if the liquids comes rushing out, then the hole is too big and the nipple should be replaced. If the bottle and nipples are discoloured, this could be a sign that the bottle is not to be used further and is deteriorating. Also, look out for cracks, chips, or breaks. Your child could cut, pinch, or otherwise injure himself. This is especially dangerous if you use glass ones.