It may simply be that his stomach is empty and the bile is causing him to throw up yellow foam. But there could be a medical condition at play, too. read more
The yellow foam is bile and the dog usually vomits bile due to an empty stomach i.e. in the morning/evening it the dog is fed once a day. More common in older dogs. You could try to feed you dog more often. read more
In most cases, yellow foam indicates that the dog's stomach is relatively empty of food. The yellow color comes from bile, a digestive fluid that's produced in the liver, stored in the gall bladder and then released into the small intestine just below the stomach. read more