Define "line of demarcation" in hair coloring.

The term "line of demarcation" specifically refers to the visible boundary that appears between newly colored hair and hair that has not been colored. This line becomes noticeable when a person has roots that are a different color from the ends of their hair, which could occur due to new hair growth or fading of previous color. Understanding this concept is essential for cosmetologists because it indicates the need for touch-up applications to achieve an even color throughout the hair. This helps maintain a seamless appearance and ensures that clients are satisfied with their hair coloring services.

The other options deal with different aspects of hair but do not pinpoint the specific phenomenon of the line formed between colored and uncolored strands, which is why they do not accurately define "line of demarcation."