One produces pungent brown seeds, which have a sharp, spicy-hot flavor and are used in Dijon mustards; the other yields milder, yellowish-white seeds used to make traditional American mustard. Purchase: Since the mustard plant is grown all over the world, seeds of both colors are widely available. Look for them in the spice aisle of your market. (You also may find ground or powdered mustard seeds.) Use: The seeds' spiciness makes them choice additions to rubs and seasonings. They're also used for pickling.