Final answer:
The innermost layer of hair, known as the medulla, consists of round cells and forms the core of the hair, which hair color does not typically reach.
Step-by-step explanation:
The innermost layer of hair, often unreachable by hair color, is called the medulla. It is composed of round cells and forms the central core of the hair. Surrounding the medulla is the cortex, which is made up of compressed, keratinized cells and is responsible for the hair texture whether straight or curly. An external hard layer called the cuticle protects these inner layers. These components are produced by the basal cells of the hair bulb, which undergo keratinization as they are pushed outwards to form the shaft of the hair growing from a hair follicle located in the skin's dermis.