What that yellow liquid actually is
The yellow or yellow-green fluid your dog brings up on an empty stomach is bile mixed with stomach fluid. Bile is made for digestion and normally moves through the system with food; when a dog goes many hours without eating — typically from an early dinner through to a late breakfast — fluid can pool and wash back into the stomach, irritating its lining until the dog vomits it out. That is why the classic pattern looks so specific: a dog who seems fine in himself, brings up a small pool of yellow foam early in the morning, then eats breakfast happily and is normal for the rest of the day. The timing and the empty stomach are the clues.
Why the evening snack often settles it
A small snack late in the evening gives the stomach something to work on and absorbs fluid that would otherwise sit irritating an empty lining, effectively shortening the fast by several hours. Keep it genuinely small and plain — a little of his normal food set aside from the day's ration is ideal, so total daily food does not creep up — and give it as late as is practical, close to bedtime rather than with dinner. Many owners see the morning episodes stop within a few days. Moving breakfast earlier can help for the same reason, and some dogs do best with both changes: last food later, first food earlier, and no long stretch in between at either end of the night.
Track the pattern before you judge the fix
Write down when each episode happens, when the last food of the evening and the first food of the morning were given, and whether the snack change altered anything over a week or two. A pattern that clearly follows long gaps and disappears when the gap shrinks supports the empty-stomach explanation. A pattern that ignores your changes, spreads into the day, or starts including undigested food is telling a different story and should not be managed at home indefinitely. Photographing an episode sounds grim but genuinely helps a vet judge colour and amount far better than a description from memory days later. Also note his overall picture alongside the episodes: a dog who holds his weight, drinks normally, and is bright between episodes fits the simple pattern far better than one who is quietly going off his food or losing condition, and those background details help a vet judge how urgently he needs to be seen.
The point where a vet needs to see him
Book a vet visit promptly if the vomiting becomes daily despite the shorter fast, if there is blood or a coffee-ground appearance, if he stops eating, loses weight, has diarrhoea alongside it, seems painful or unusually tired, or if he is a puppy, a senior, or managing another condition. Morning bile vomiting is common and often simple, but it shares its main sign with a list of problems — stomach inflammation, dietary intolerance, obstructions, and organ issues among them — that only an examination and sometimes tests can separate out. Bring your notes: when each episode happened, feeding times, what the snack trial changed, and recent food or routine changes. KinsLe's Pet Passport keeps that episode log and feeding history together in one portable packet, so the vet sees the whole pattern at the visit instead of a reconstruction.