Final answer:
Permanent haircolor requires a patch test to ensure there is no allergic reaction or skin sensitivity to the product. It must be conducted 24 to 48 hours before the full application and can prevent potential adverse reactions on a larger scalp area.
Step-by-step explanation:
Permanent haircolor requires a patch test 24 to 48 hours before application. This patch test is crucial for identifying any potential allergic reactions or skin sensitivities to the dye's ingredients. To conduct a patch test, a small amount of the haircolor product is applied to the skin, usually behind the ear or on the inner forearm, and left to dry. After 24 to 48 hours, the area should be checked for any signs of irritation or allergic response, such as redness, itching, or blistering. If any adverse reaction occurs, the haircolor should not be used. By performing this test, you minimize the risk of experiencing a severe allergic reaction when you apply the dye to a larger area of your scalp.