Hello again, and thanks for your additional information.
Green stools can indicate the ingestion of a toxic substance or non-food item, or can also indicate excess bile or acid in the stomach. If your dog has had any access that you know of, to rat poison, the pellets are sometimes neon green, so keep this in mind.
If the green stool occurs, along with vomiting and/or diarrhea, this may indicate symptoms of Inflammatory Bowel Disease. If not, then there are a variety of other reasons why the stool may be this color.
A dog's stool may be affected by things they are eating. Certain additives in your dog's current food, mix with the bile in his stomach, and in turn, create anywhere from a greenish to a definitely green stool. Sometimes, a daily dose of Pepcid AC will cut down on the amount of stomach acid, but always check with your vet before giving any over the counter medication to your dog.
Even if your dog is acting normally, monitor what he's eating. Try changing his diet, gradually, to a food with no artificial additives/preservatives or corn, gluten, soy, dairy products, as these are common allergens for dogs, and see if that helps. Some good premium brands of dogs foods which contain no allergens are Wellness, Canidae, Solid Gold, Newman's Own Organic, and Natural Balance.
Your dog may be eating plants (indoors, or the grass under the snow, outdoors) or other things that may be affecting his stool. Some plants are toxic to dogs, so be careful. If his stool becomes more watery and/or he's not eating as usual, acting lethargic, a vet visit is definitely
your best course of action. Bring a fresh stool sample for analysis, and once the problem is identified, your dog will be treated appropriately.
I hope he'll be fine!