Mustard seeds are the small seeds of the various mustard plants. The seeds are about 3 mm in diameter, and may be colored from yellowish to blackish yellow. While there are approximately forty different varieties of mustard plants, there are two principal types used to make mustard seeds: black mustard, yellow mustard Black mustard seeds have the most pungent taste, while yellow mustard seeds are the most mild. Yellow Mustard seeds or mustard powder can be used as a condiment in a variety of dishes.