Final answer:
To enhance the natural color of a medium brown level 4 shade hair, an orange-red base should be used in the hair color product, complementing the natural pigments of the hair.
Step-by-step explanation:
If a client's hair is a medium brown level 4 shade and she wants to enhance her natural color, the base of her hair color product should most likely be orange-red. This is because, in hair coloring, level 4 corresponds to a medium brown, which has a natural base color of orange-red pigments. These pigments are what give the medium brown its warmth and depth. When enhancing natural hair color at this level, using a color with an orange-red base will complement and amplify the existing natural pigments.
Dyes contain couplers, which are chemical compounds that define the color of the hair dye. Couplers can be red, yellow-green, or blue, as demonstrated in the color chart provided as Figure 21.6.11. When added to a dye, these couplers, through a chemical reaction facilitated by the oxidation of p-phenylenediamine to the quinonediimine, interact to create the final hair color.