Which of the following is NOT a vital sign?

Vital signs are clinical measurements that provide important information about a person's health status. They are used by healthcare professionals to assess the basic body functions and can indicate the physiological state of an individual. The primary vital signs include pulse, body temperature, respiration rate, and blood pressure.

In this context, pulse, body temperature, and blood pressure are all considered essential vital signs. The pulse reflects the heart rate and rhythm, body temperature indicates the core body heat, and blood pressure measures the force of blood against the walls of the arteries. Each of these metrics is critical in evaluating a person's immediate health conditions.

Skin color, however, is not classified as a vital sign. While changes in skin color can provide information regarding a person’s circulatory and respiratory status, it does not have a standardized measurement method or threshold that can be quantified like the other vital signs. Therefore, while skin color can be a useful observational indicator in assessing a patient's condition, it does not fall into the category of vital signs as defined by clinical standards. Thus, identifying skin color as not being a vital sign is correct within the context of this question.