Is dark green diarrhea bad? I ate berry crunch cereal 3 times in the last 12 hours.

Maybe. Maybe not. Having occasional episodes of vomiting or diarrhea can be due to something in the food (such as food poisoning by bacteria or their secreted chemicals). Another cause of such symptoms is a stomach virus. A primary care doctor can review one's history and do an exam to see if there is anything to worry about. Symptoms that go away the next day and don't return, are usually not a worrisome problem.