Parents must always consider in advance what message their children will "hear" from the response they have to their child’s behavior. They also must pick their battles. By this we mean looking at each situation or behavior and weighing it on a scale of importance.
Small children go through phases of things they like to eat, wear and do. It typically does not last long, and it seldom causes serious damage to their health or home. Our goal is not to control our children, but to teach them self-control.
This situation with the socks is something that could be written in your granddaughter’s baby book and be remembered with amusement for years to come. This issue is simply not worth fighting the child over.