Mustard Seeds | Large, both white and brown, are nearly globular in shape, finely pitted, odorless when whole, and pungent-tasting. White mustard seeds are light yellow in color and about 2.5 mm (0.1 inches) in diameter. Brown mustard seeds are about the same size but are a darker yellow in color. What are mustard seeds used for? But mustard seed works equally well to kick up the flavor of mild ingredients such as potatoes, legumes, chicken, and seafood. Add it to the dressing for potato, cucumber, or chicken salad. Use it in rubs, marinades, and sauces, and include it in curries and bean dishes.