What is the function of a toner in hair coloring?

The function of a toner in hair coloring is primarily to adjust the tone of the hair color. Toners are typically used after the hair has been lightened, whether through bleaching or highlighting, and they help to refine or enhance the shade of the hair. For instance, they can neutralize unwanted brassiness or yellow tones, resulting in a more desirable hue, such as ash or pearl.

By using a toner, stylists can achieve a more balanced and polished final color, allowing for variations in shade that might not be achievable with the initial dye alone. This process highlights the importance of toners in achieving the perfect color result that meets the client's expectations. The other choices presented do not accurately describe the role of toner; they pertain to different aspects of hair care and coloring, such as lightening, nourishment, and protection from environmental damage.