What is the main function of a toner in hair coloring?

The main function of a toner in hair coloring is to neutralize unwanted undertones. When hair is lightened, it can sometimes reveal undertones that may be undesirable, such as brassiness or overly warm shades. A toner is specifically designed to adjust the color by introducing pigments that counteract these unwanted tones. For instance, if hair turns too yellow after being lightened, a purple toner can help neutralize that yellow, resulting in a more balanced and pleasing color.

The other possible functions, such as lightening hair, adding shine, or conditioning, are not the primary purposes of a toner. While some toners may have conditioning properties, their main role is focused on color correction rather than altering the hair's lightness, enhancing shine directly, or conditioning the hair. Hence, using toner effectively requires an understanding of color theory and the desired outcome for the hair.