Final answer:
To add violet to a level 3 dark brown hair, the base of the color product should be red, as violet comprises red and blue and red couplers help in achieving the desired hue.
Step-by-step explanation:
If a client is at a dark brown level 3 and wishes to add some violet to their hair, the base of the hair color product should be red. Violet in color theory is a combination of blue and red, but given the level 3 base, adding more red will counteract the darkness of the brown to bring out a violet hue. In hair coloring, couplers define the color of the dye, and for violet, adding a red coupler would be the appropriate choice.
Permanent hair coloring involves a complex chemical process. Essentially, it combines an oxidant, a coupling agent, and a substance like 1,4-diaminobenzene or its derivatives. The first stage of this process involves the oxidation of a substance, typically 1,4-diaminobenzene, to form a quinone. Next, this reacts with a coupler compound to form a new complex. Finally, additional oxidation steps result in the final dye color.