Which technique is used to darken hair color gradually?

The technique that is used to darken hair color gradually is toner application. Toners are semi-permanent or demi-permanent color products that can add depth and richness to the hair without a drastic change. They are particularly useful for adjusting the tone of already lightened hair, as they can neutralize unwanted warmth or enhance certain shades.

Using a toner allows for a more controlled and subtle change in color, making it an ideal option for clients looking to enrich or darken their hair over time rather than in one dramatic session. This gradual approach is beneficial for maintaining healthy hair, as it mitigates the potential damage that can come from more aggressive processes like bleaching or color stripping.

Other techniques like bleaching and color stripping are intended to lighten hair color significantly and quickly rather than darken it. Highlighting adds lighter strands to the hair instead of darkening overall color, further distinguishing its purpose from toner application. Thus, toner application serves as the correct method for those looking to achieve a gradual darkening of their hair color.