yellow mustard seeds have a yellow color while black mustard seeds have a much darker exterior. They aren’t actually black. Their color is typically dark brown tinged with red.

Yellow mustard seeds and black mustard seeds come from two different plants in the Brassica family. Yellow mustard seeds come from the Brassica alba plant while black mustard seeds come from the Brassica nigra plant.

Another key difference is flavor. Yellow mustard seeds are considered the mildest of all the mustard seeds. They are used in American ballpark mustard, which is better known for its vinegary tang than for heat. Black mustard is at the other end of the spectrum as it is particularly pungent and hot.

Yellow mustard seeds are much easier to find in grocery store spice aisles than black mustard seeds. Black mustard seeds are still a specialty item. Look for them in markets that carry Indian and Asian products. The reason in availability is that yellow mustard seeds can be picked using machines while black mustard seeds must be picked by hand making the process of harvesting them more labor-intensive and expensive.

Can you use yellow mustard seeds in place of black mustard seeds and vice versa?
Yellow mustard seeds do have a mustardy tang, so they can provide some flavors that you would get from black mustard seeds. What they won’t provide is heat, which is usually the main reason that people use black mustard seeds. 

Black mustard seeds can replace yellow mustard seeds in many dishes but are best when you want to give your dish a little heat. If you prefer mild flavors and black mustard seeds are your only option, you can try taming the heat with a little vinegar or using fewer seeds. Keep in mind that to preserve their pungency, you will need to add black mustard seeds late in the cooking process.

When should you use yellow mustard seeds and when should you use black mustard seeds?
Use yellow mustard seeds when you need a mild mustard flavor. If you plan on making your own American yellow mustard or want to dilute the heat in prepared brown or black mustard, yellow mustard is a good option. It works best when you need the mustard flavor without the bite.

Black mustard is your best option for pungency and heat. If you are cooking Indian food, this is the kind of mustard you need. Traditional Indian applications include curries and a range of spicy Indian dishes where the seeds are often tempered in oil or ghee until they pop. It is a common ingredient in some versions of Indian spice blends like panch phoron and cambaar podi. If you want an especially hot prepared mustard, black mustard seeds will be your best bet.