How can a barber remove the line of demarcation when coloring hair?

Removing the line of demarcation involves addressing the noticeable contrast between colored hair and new growth that may be a different color, typically the natural hair color. Applying permanent hair color to the new growth effectively blends the roots with the rest of the colored hair, thereby softening or eliminating the line of demarcation. This method ensures a seamless transition, allowing for a more natural appearance and continuity in hair color.

In this context, the other options do not specifically address the issue as effectively. Cutting the hair shorter doesn't blend the colors; it merely alters the length. Using a lighter shade on the whole head can create an uneven color result, especially if the previously colored hair is darker, leading to further contrast rather than resolving the demarcation. Recommending a color treatment alone doesn’t necessarily resolve the line unless it involves applying color to the roots, leaving the problem unaddressed without specific action taken on the new growth. Thus, applying permanent hair color to the new growth stands out as the most effective and direct solution.