Final answer:
The process of treating gray or very resistant hair to allow for better penetration of color involves using a base color with permanent highlights.
Step-by-step explanation:
The process of treating gray or very resistant hair to allow for better penetration of color involves using a base color with permanent highlights. Gray and white hair tend to be more resistant to color absorption, so using a semi-permanent color alone may not be enough. By incorporating permanent highlights with a semi-permanent base color, the hair is pre-lightened which allows for better penetration of the color and helps to blend in the gray or white hairs more effectively.