Hello,
It sounds like your dog is vomiting bile. Sometimes if a dog hasn’t eaten in awhile the bile can irritate the stomach and then they vomit. How long does your dog go between meals? If your dog only eats once a day, try doing twice per day…or even three times per day if possible. Use the same daily amount of food, just divide it up into more than one feeding.

 Of course, it could be other things that can cause this, but this would be my first thought.
~kelly