Final answer:
A toner can balance out overlightened hair, yet if hair becomes too porous, the toner might lead to undesirable effects. It's used to achieve natural-looking colors but must be applied carefully.
Step-by-step explanation:
If the hair is overlightened, a toner may make the hair appear to have a more even tone by adding in pigments to balance out the overly lightened hair. However, if the hair is excessively lifted and becomes too porous, the toner can result in undesired effects like an ashier, duller, or even a slightly different color than intended. Toners are often used after bleaching to achieve a more natural-looking hair color, but if not used carefully on hair that is overlightened, the result may be less than ideal.