The peppery leaves of the mustard plant are a rich dark green and have a pungent mustard flavor. They belong to the same family of vegetables as collard greens, broccoli, Brussels sprouts, kale and kohlrabi. A popular "soul food" ingredient. Selection: Look for crisp young leaves with a rich green color. Reject those with yellow or pitted leaves or thick, fibrous stems. Storage: Refrigerate greens in a plastic bag for up to a week.