If you notice that your dog throws up yellow mucus, then it is a sign that your dog is suffering from some health issue. Bile is a fluid produced in the liver and stored in the gallbladder, which enters the duodenum to further assist with digestion. When dogs vomit yellow fluid, it may simply be that the stomach is empty or your dog is suffering for Gastrointestinal Disorders, food allergies, or eaten grass. Other possible reason a dog throws up yellow mucus includes lack of food, drinking a lot of water, lack of fibrous food, certain medication, allergies, eating fatty food or Bilious Vomiting Syndrome