What aspect of hair does the cortex primarily influence?

The cortex of the hair is the middle layer of the hair shaft and plays a crucial role in determining the hair's color. It is responsible for the pigment that gives hair its color, as it contains the melanin granules. Different types and amounts of melanin result in various hair colors, ranging from black to blonde to red. The structure and composition of the cortex also affect how light interacts with the hair, which can further influence its perceived color.

While the growth rate, moisture retention, and texture of hair are influenced by other factors, such as the hair follicle's health, scalp condition, and overall hair care practices, the cortex specifically influences the color of the hair itself through its pigmentation. This makes it the most relevant aspect among the options presented.