This is Dr.Christian. The yellow fluid you are seeing is called bile. It is produced by the liver and is then used in the small intestine to digest food. When a puppy is vomiting bile it means that the stomach is empty and the vomiting is now starting to empty the small intestine. In this case it would definitely be be best to have your puppy checked by a vet immediately if you have not already done so. If you have more questions please let me know.