Mustard comes in many varieties and flavors, and there can be, as a recent blind tasting proved, an equally wide range of tastes and colors even within a specific type of mustard.
Ditto for the wording on the labels, which can read "Düsseldorf," "Düsseldorf style," or say nothing at all, even when the mustard is made in Düsseldorf. You may need to linger in the condiments aisle and read labels to get some idea of what you're buying.
Tasters sampled a variety of mustards in the Düsseldorf style. Four were made in the United States, the fifth in Germany.
Look for most of these mustards in your area supermarkets or gourmet grocers. All but the White Castle mustard are available via the online store ( store.mustardmuseum.com) of the National Mustard Museum in Middleton, Wis. Prices below are from the museum's website. The White Castle Düsseldorf mustard is sold in the chain's restaurants, but only in certain regions, including the Midwest; telephone a local franchise to check availability.