Final answer:
To transition from a medium red-brown to a lighter hair shade, it is essential to remove the present tint first, followed possibly by the application of a filler or protein treatment before applying the lighter color.
Step-by-step explanation:
To change your client's hair from a medium red-brown to a lighter shade, removing the present tint is necessary before applying a new color. This is because the existing tint can affect how the new color will look on the hair. To remove the present tint, a hair color removal product, which will strip the hair of its current color, is often used. It's essential to follow this with a thorough washing and potential conditioning treatment to prepare the hair for the new tint. After the tint has been removed, you may need to apply a filler or protein treatment to ensure that the hair can evenly absorb the new, lighter color.
It is critical to understand that the chemistry of hair coloring involves complex reactions. For example, the oxidation of p-phenylenediamine leads to the formation of quinonediimine intermediates, which then react with other compounds in the hair dye like couplers (e.g., red couplers, yellow-green couplers, blue coupler) to form the final color on the client's hair.