Which technique is used to tone or darken the existing colour along the entire strand?

The technique used to tone or darken the existing color along the entire strand is known as the base to ends technique. This method typically involves applying color from the roots through to the ends of the hair, ensuring that the entire strand is uniformly treated. It is particularly effective when the goal is to achieve a richer or darker shade, providing a consistent look throughout the hair.

In a salon setting, this technique is often utilized during a single-process color application, where a colorist aims to modify the existing shade all the way down to the tips. It allows for a seamless transition and maintains overall hair health, making it a popular choice for clients looking for a subtle yet significant change.

Other techniques mentioned, such as highlights and balayage, focus more on adding dimension or lighter colors rather than darkening the entire strand. Lowlights, while they do darken certain strands, are typically used to add depth and dimension instead of modifying the entire strand color uniformly. Thus, the base to ends technique stands out as the most appropriate method for toning or darkening color thoroughly.