Final answer:
The part of the hair shaft between the scalp and the hair that has been previously colored is the regrowth. It is the new growth from the scalp without any chemical color treatment which consists of three main layers: the cuticle, cortex, and sometimes the medulla.
Step-by-step explanation:
The part of the hair shaft between the scalp and the hair that has been previously colored is known as the regrowth. This refers to the new hair that has grown from the scalp since the last hair coloring treatment and has not been subjected to any chemical color change. It is part of the hair shaft, which is composed of dead, keratinized cells and includes three main layers: the cuticle, cortex, and sometimes the medulla.
The cuticle is the outermost layer, the cortex is the middle layer responsible for the texture and shape of the hair, and the medulla is the innermost core which may not be present in all hair types. When coloring hair, it is the cuticle layer that is first opened in order to allow color to penetrate into the cortex. As the hair grows, the new hair, which comes from the root within the follicle, appears without the artificial coloring and is known as the regrowth.