Final answer:
Yes, a client with black, previously color treated hair will need a lightener to go 6 shades lighter, as hair color alone cannot lift the hair to that extent and bleach is needed to break down melanin.
Step-by-step explanation:
If a client has black hair that is already color treated and wants to go 6 shades lighter, yes, they would need a lightener. Black hair contains a high concentration of melanin and to lighten the hair by several shades, the natural pigment needs to be lifted. Hair coloring alone cannot lift previously colored hair enough to reach 6 shades lighter without the help of a lightening product, which breaks down the pigments. Hairdressers typically use a bleach or other professional lightening products to achieve such a significant change in color. It is a chemical process that requires expertise to minimize damage and maintain the health of the hair.