If the dog is acting fine otherwise I wouldn't be too concerned. Sometimes vomit can appear this way with no real emergency - just digestive upset. There is some concern that if a dog has a chronic vomiting problem it can be a partial blockage in the stomach or intestine. This can be the case also when the vomit appears a little stool-like. This can be because the food is only able to pass so far through the intestine and then comes back up. It is possible that if there is a partial blockage it only occurs with things like the bone that can't pass by. I would stop giving the bones to see if the vomiting still occurs. If there is no vomiting without any bones it may just be that these are upsetting his system. If there is a partial blockage there should still be some vomiting to let you know that there is still something to be concerned about. Other signs to watch for are a tender abdomen and diarrhea. Here is a link to more information about intestinal blockages.
If the dog is acting fine, it is probably not a medical emergency, and you can treat them at home for intestinal upset for a bit. This usually involves fasting the dog for 12 - 24 hours and then feeding a bland and bulky diet of boiled chicken and rice (2/3 rice) for a few days. Then you take another couple of days to transition back to normal diet. If he is or becomes lethargic, painful, refuses to eat anything, is unable to keep anything down, or the vomiting returns after a couple of days I would suggest a trip to the vet. I would also take him to the vet if he continues to vomit occassionally with no bones in his diet. They will probably want to do x-rays to check for blockages. If he continues to vomit like this you can bring it into the vet when you bring him in so that they can see what you are talking about.
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I have definitely seen blockages caused from various types of bones. I still give them to my dogs however. I just watch them with them and let them chew on them until they start to get pieces off and then take them away. It is a risk that you are taking though. I also do not give my dogs bones that have been boiled or used for soup as these are softer and easier to get pieces off of. I also only give them larger beef bones (poultry bones are quite dangerous and pork doesn't sit well with a lot of dogs) Some dogs though with sensitive stomachs or that tend to be very heavy chewers that can get pieces off of bones quickly or eats them in large pieces should never have bones. The safest answer would be to not give them to your dog as then you can avoid any of these possibilities.
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