Which tool is primarily used for foiling in hair coloring?

The tool that is primarily used for foiling in hair coloring is foil sheets. These sheets are crucial because they provide a barrier that allows the stylist to apply lightener or color to specific sections of hair without the mixture bleeding into other areas. Foil sheets facilitate the application of color while also reflecting heat back to the hair, which can enhance the processing time and overall effectiveness of the color application.

In the context of foiling, the other tools like brushes, combs, and sponges serve different purposes. Brushes are typically used for applying the color or lightener, while combs might assist in sectioning the hair or detangling. Sponges are generally not used in the foiling process for coloring hair, as their application method and texture do not lend themselves well to this technique. The specific design and function of foil sheets make them the most suitable tool for achieving the precise and clean application required in foiling hair coloring.