Darkening hair is a process that can be done for a variety of reasons, including to cover gray hair, to add depth and richness to natural hair color, or to create a new, more dramatic look. There are a number of different ways to darken hair, including using chemical dyes, natural dyes, and henna.
Chemical dyes are the most common way to darken hair. They work by penetrating the hair shaft and depositing color molecules. Chemical dyes can be permanent or semi-permanent. Permanent dyes last for several weeks or months, while semi-permanent dyes last for a few weeks.