Mustard seeds are the small round seeds of various mustard plants. The seeds are usually about 1 or 2 mm in diameter. Mustard seeds come in different colours. The black seeds are very pungent. White seeds tend to be milder but can have the fieriness of black. Black or brown seeds are used widely in Indian cuisine. Yellow mustard seeds are milder and commonly used in European and American cooking and widely used in making whole grain and Dijon mustards.