Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that Hobo has been vomiting for 2 days, the contents of which now look green.
Cats that have a gastrointestinal blockage or very abnormal intestinal motility often vomit green material as that is digested bile, bile that sits in the stomach for a while. Bile only refluxes back into the stomach when gastrointestinal motility is abnormal, such as with a blockage or poor gastrointestinal motility due to primary intestinal or metabolic disease.
Vomiting can be related to chronic pancreatitis, esophageal reflux, a dietary allergy or sensitivity, toxin exposure, a viral or bacterial infection, inflammatory bowel disease, heartworm disease, hyperthyroidism (a tumor of the thyroid gland), internal organ failure, a full or partial gastrointestinal obstruction or even infiltrative cancers such as lymphoma.
Is he a kitty that gets outdoors unsupervised such that he could get into something you don't know about?
Or is he a kitty that eats things like rubber bands, toys other things he should not?
Because he isn't eating or drinking I am very concerned about him. He will quickly become dehydrated and as his liver breaks down fats for energy to live he may develop a type of liver disease called hepatic lipidosis, which can also be deadly.
If he does go outdoors exposure to infections, toxins, and eating things he should not are real concerns.
Ideally he would see a veterinarian since this has been going on for more than 48 hours. Simple stomach upset should pass within 24 to 48 hours. They could examine him, run blood tests and possibly check radiographs and/or an ultrasound to evaluate him and know best how to treat him.
In the meantime they can administer injectable anti-nausea drugs and fluids to rehydrate him.
If you cannot have him seen by a veterinarian right away for whatever reason there are some things you can try at home, though if he doesn't respond quickly he should see a veterinarian.
At home to try and settle his stomach you can give either:
1) Pepcid ac (famotidine) at a dose of ¼ of a 10mg tablet per 5 to 10 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at a dose of ¼ of a 10mg tablet per 5 to 10 pounds of body weight every 24 hours
These are acid reducers and should help him feel better, hopefully stop any further vomiting, and help him be able to drink and keep fluids down. They are quite safe and can be used for several days if necessary.
You can use a medicine syringe to try and force water into him orally a couple hours after giving an acid reducer if he continues to refuse to drink.
A couple of hours after giving the acid reducer I recommend offering soft foods mixed with water to make eating and swallowing more comfortable.
If she is refusing to eat even soft foods you can try offering meat baby foods or a bland diet of 2/3 boiled minced white skinless chicken and 1/3 boiled white rice mixed with some low salt chicken broth to make it easy to lap up and swallow.
If Hobo continues to not eat and continues to vomit green material he should see his veterinarian or an emergency veterinarian for an examination, diagnostics and intravenous fluids and supportive care.
Please let me know if you have any further questions.