You can also store a comment about the material.
The colouant database is stored in the Ram Materials folder.
Bentonite is not a "Colourant" but it is added to a recipe in small quantities to aid with suspension. This material does not change the base recipe chemistry significantly so it is included in the colourants database.
Zirconium Silicate (often known by its commercial name "Ultrox", "Zircopax", etc) is also a material that does not add colour. It is an opacifier as is tin oxide. These are added to a recipe but do not change the chemistry of the base glaze. They are also therefore included in the "colourants" listing.