Where should you apply a single process root touch up during hair coloring?

Applying a single process root touch-up requires precision to ensure a seamless and uniform color throughout the hair. The correct approach is to apply the color up to the line of demarcation. This line represents the point where the previously colored hair meets the new growth. By focusing on this area, you ensure that the new color blends effectively with the previously colored hair, thus achieving a smooth transition without any visible gaps or harsh lines.

Using this method also helps maintain the integrity of the previously colored hair, as applying color to the entire length (as suggested by another choice) could lead to unnecessary damage and over-processing. Additionally, applying color only to damaged areas would not address the need for even color application across the roots, while focusing solely on the tips of the hair strands would not effectively touch up the roots where the new growth is most visible.