Your dog is likely throwing up yellow because of stomach bile. This is a digestive fluid produced by the liver and stored in the gallbladder that helps break down fats in the small intestine. When a dog's stomach is empty for a long time, bile can back up into the stomach and irritate its lining, causing vomiting of yellow bile or foam. This condition is sometimes called bilious vomiting syndrome and often occurs in the early morning or late at night after a long gap between meals. Other possible causes include:

• Eating something that doesn't agree with them, like grass or garbage
• Gastrointestinal inflammation or disease, such as gastritis or pancreatitis
• Blockages in the digestive tract from foreign objects or tumors
• More serious health issues like liver disease, food allergies, infections, or cancer

If the vomiting happens occasionally and your dog is otherwise normal, feeding smaller, more frequent meals often helps. However, if vomiting is frequent or accompanied by other symptoms like lethargy, diarrhea, abdominal pain, or refusal to eat, a veterinary evaluation is important to rule out serious conditions. Overall, yellow vomit usually indicates stomach bile, commonly related to an empty stomach or digestive irritation, but underlying health conditions should be considered especially if symptoms persist or worsen.