What is the ruling on women using chemical dyes, especially what is known as "mesh" (highlights)?

The basic principle regarding women's adornment is permissibility, except for what is prohibited by evidence. A woman dyeing her hair is permissible as long as there is no Sharia-based impediment, such as imitating disbelievers, or if it is with pure black dye—according to what we issue fatwas on—or with a harmful substance. "Highlights" are permissible.