Final answer:
For a single-process retouch application, the color should be applied to the new growth using an on-scalp application method to ensure consistent root-to-end color.
Step-by-step explanation:
When applying color for a single-process retouch application, the color should be applied to the new growth using c. On-scalp application. This method involves applying the color directly to the new hair growth at the scalp, ensuring consistent color from the root to the end of the hair. A balayage technique is more suited for freehand painting highlights onto the hair, creating a more natural, sun-kissed look. Foil highlights are used for precise, even highlights through sectioned hair using foils. Lastly, Off-scalp application refers to color techniques that are applied away from the scalp, such as on the lengths or ends of the hair, rather than at the root.