Read here for future reference:
Why is My Dog Vomiting Yellow Liquid?: Causes for this Chronic Form of Canine Stomach Upset It explains that a dog will vomit yellow bile when its stomach is empty OR when there is excess stomach acid. My JR often vomits yellow bile and it is more the excess acid than no food in stomach. I have to make her stay calm after eating as she will vomit for sure if she gets rowdy after eating. I can also give her some Pepcid if needed (only with vet's permision!). The article is pretty good at explaining this problem, potential causes, and treatment.