Red underlayer hair dye, a semi-permanent or permanent coloring agent, is applied underneath the top hair layers to create a subtle, peek-a-boo effect when styled. For instance, a vibrant burgundy shade beneath blonde or brown hair adds depth and dimension.
This technique is rising in popularity due to its low-maintenance nature and ability to enhance natural hair tones. Red underlayers have been used for centuries, with ancient Egyptians employing henna to add a reddish hue to their hair.