What is the process of treating gray or very resistant hair to allow for better color penetration called?

The process of treating gray or very resistant hair in order to facilitate better color penetration is known as presoften. This technique involves applying specific products or treatments that help to open the hair cuticle, making it more receptive to hair color. Gray hair and resistant hair types often have a denser cuticle layer, which can hinder the absorption of color. By presoftening the hair, the color can penetrate more effectively, leading to a more even and vibrant result.

The other options do not pertain specifically to preparing hair for color application in the context of gray or resistant hair. Retouch refers to the application of color to the regrowth of previously colored hair, which does not address the issue of penetration. Progressive dyes involve a buildup of color over time rather than a treatment for penetration. Semi-permanent hair coloring is a type of dye that does not require a developer and generally does not involve any preparatory treatment like presoftening.