Mustard seeds can be white, yellow, black or brown, and are derived from three different plants. The black seeds are exceedingly pungent; they’re also difficult to harvest, volatile and thus more expensive. White seeds tend to be much milder but can have the fieriness of the black, depending on how they’re prepared. Black or brown mustard seeds are widely used in Indian, especially Bengali and Southern Asian, cooking. When fried, the taste is nutty rather than fiery.