Rotkohl and Blaukraut are often used synonymously, but they are actually two different dishes. Rotkohl always has among its ingredients an acid (vinegar), which keeps the cabbage red. Recipes for Blaukraut do not include an acid, which results in a much bluer end product. (Science!) Like Hot German Potato Salad, Rotkohl may be served hot or cold and is even better the next day.