Hair color under hair, also known as underpainting, is a coloring technique that involves applying a darker shade of hair color to the hair's underlayers, creating depth and dimension to the overall hair color.
This technique can enhance the vibrancy and richness of hair color, making it appear more natural and less flat. Underpainting can also help to correct uneven hair color or cover up unwanted tones. Historically, it was used as a way to create the illusion of thicker, fuller hair.