Brown and yellow mustard come from the same family, but different varieties of the plant. The brown mustard seed is smaller and hotter than its yellow counterpart, due to its high oil content. Yellow mustard powder is great for creating your own (more intensely flavored) mustard. Traditionally, yellow is used to make mild mustards but can add flavor to any dish.
Spice Fun Fact: The idiom, cant cut the mustard, is most often used in the negative form, meaning that one cant live up to a challenge or expectations.