First,check to make sure it is not just dye from the food you feed him. The brown coloration is an indication of blood or bleeding from the digestive system.
If your dog is vomiting regularly, he needs to be seen by a veterinarian immediately. There are dozens of serious reasons for vomiting, regardless of the color.
This is not serious one as dogs do that always if they have some issues with their stomach. Also the food that it ate the day before could be allergic.