What is grey salt used in cooking?

There's no such thing as "grey salt" in the culinary world. It's likely you're thinking of one of the following:

* Grey Sea Salt: This is a popular type of sea salt that has a distinctive grey color due to the presence of minerals like clay and volcanic ash. It has a slightly more complex flavor than regular table salt and is often used to finish dishes.

* Fleur de Sel: Also known as "flower of salt," this type of sea salt forms in thin, flaky crystals on the surface of salt pans. It has a delicate, slightly sweet flavor and is often used as a finishing salt. It can have a slightly grey hue, but it's not always grey.

* Sel Gris: A type of grey sea salt from France. It's similar to Fleur de Sel in terms of its texture and flavor, but it can have a more intense, slightly briny taste.

If you're interested in learning more about different types of salt and their uses in cooking, there are many resources available online and in cookbooks.