What does a color retouch typically involve?

A color retouch primarily focuses on maintaining or refreshing the existing color of the hair, particularly at the roots where natural growth occurs. Over time, new hair growth can lead to visible roots that differ in color from the rest of the hair, making a color retouch necessary to achieve a uniform look. This process typically involves applying the same color to the roots to blend seamlessly with the previously colored lengths, ensuring a consistent overall appearance.

In contrast, the other options describe different techniques or applications. Applying color from roots to tips relates to a full color application rather than just a retouch. Color application only to the ends is more relevant to a technique known as a "refresh" or "gloss," which may be done to brighten the ends. Lastly, applying lightener to all of the hair suggests an overall lightening process, which is distinct from merely retouching the roots.