There are three types of mustard seed to look out for when
shopping. The most common, Stateside anyway, is the plain jane mustard seed,
pictured above. It's my favorite for pickles, meat rubs, and European and
Western dishes like Boston Baked Beans. Yellow mustard seeds have a balanced, not-too-intense flavor, and
their light color makes them easy to cook when heating them in hot oil (as
their color darkens, you can pull them off the heat before they turn black and
burn).
If you like your mustard with more kick, or have Indian cuisine
set in your sights, brown/black
mustard is the way to go. It's the same size as common
yellow mustard seeds but with a
more intense flavor that tastes more mustardy. Whole-seed
deli mustards and dijon mustards use black seeds to get their piquant punch. I
use these in all manners of curry, lentil soups, sautéed potato dishes, and
roasted vegetables, especially when blended with other spices.
Less well-known, but worth having around
are hot mustard seeds, also
called "Chinese" or, if your merchant isn't feeling especially PC,
"Oriental." Hot mustard seeds are much smaller than yellow or brown
varieties, though they pack quite a wallop. To my taste, they're hotter but
less interesting than brown mustard seeds. I like them for quick stir fries,
especially with greens, where there aren't other spices to get in the way.
While brown mustard seeds contribute complexity to spice blends, I think hot
mustard seeds are best for adding zip to milder ingredients, like spinach.