I had a feeling it might be from the plants outside. It's been raining recently and his bowl sits right under the bushes. Could it be possible that the rain runoff from the plants could make him sick? I changed to a different one last night and this morning his vomit was clear and foamy and he's starting to purr and move around again.
If it is yellow bile, it might be regurgitation. A problem that can be confused with vomiting is regurgitation, which is the ejection of contents of the esophagus. Food moves quickly through the esophagus to get to the stomach. If the muscle of the esophagus loses tone, the esophagus dilates (Megaesophagus). A dilated esophagus does not effectively move food to the stomach and the animal will regurgitate food usually shortly after eating.
You might want to take him to the vet. Vomiting or regurgitating food isn't fun and it is basically due to health problems. Take a sample to the vet so they can better assess what IT really is and what could be causing it. Has he been coughing?
Have you checked to see if all of your houseplants are cat safe and if not they are out of reach?