Final answer:
During a retouch service, previously processed hairstrand is not re-processed to avoid unnecessary damage. Only the new growth at the roots is targeted and colored.
Step-by-step explanation:
During a retouch service, the parts of the hairstrand that have already been processed previously will not be processed again. This is because the previously processed hairstrand has already been permanently colored and there is no need for further processing. By avoiding processing the previously colored portions, the retouch service ensures that only the new growth at the roots is targeted and colored.