Final answer:
Pre-lightening is the process used for treating gray or resistant hair to ensure better color penetration and effectiveness by using chemical lighteners such as bleach.
Step-by-step explanation:
The process of treating gray or very resistant hair for better penetration of color is called pre-lightening. This technique is often used to improve the penetration and longevity of the color applied to hair that has become resistant due to its gray nature or other factors. Unlike semi-permanent coloring, pre-lightening alters the hair's structure to prepare it for more effective dye absorption. The process typically involves the use of chemical lighteners, such as bleach, to remove the natural pigment, thus allowing the hair to become receptive to the new color.