Final answer:
The pretreatment process known as priming is used to prepare gray or very resistant hair for better dye penetration, resulting in more effective and long-lasting color.
Step-by-step explanation:
The process of pretreating gray or very resistant hair to allow for better penetration of hair dye is known as priming. In hair coloring, priming can involve the use of pretreatment products or methods that prepare the hair to better absorb the hair dye. Since gray or very resistant hair has a tighter cuticle layer, the priming process ensures that the dye can effectively penetrate this layer, leading to more uniform and long-lasting color. This step is vital for achieving desired hair color results, especially when dealing with stubborn hair types.