Mustard seeds come in three colours: yellow, brown and black. Yellow seeds are used throughout Europe for pickling and preserving, as well as ground in sauces, dressings and condiments, including, of course, mustard. Brown and black seeds are very similar, though black mustard seeds are larger and more potent. Both types are hotter and more intense than yellow. Black seeds are quite delicate and difficult to harvest, so more and more often, brown seeds are being planted and used in their place, making black seeds very difficult to find. Brown and black seeds are interchangeable and are used widely in Indian food.
For ground yellow mustard seeds, look under “Mustard Powder.”