The Wella Blondor Freelights Developer 40 Vol. (12%) is a key part of the Blondor Freelights system, designed for natural-looking, foil-free highlights. Its Actifuse Technology blends with Blondor Freelights White Lightener to create a flexible mix for precise, freehand application. This provides maximum lift. It's used for achieving significant lightening and creating bold, bright results, especially on darker hair.