If you are planning to take a predisposition test, there are some things that you have to do. The first step is to identify the patch where you are going to do the test. This is usually done behind the ear especially when you are planning to have your hair colored.
The hair color will then be applied to that area and you would wait for 24 - 48 hours to see if there would be any reaction on your skin. If there is a reaction, you will not be recommended to continue with coloring your hair. If there is no reaction, this is a sign that the product is safe to use for your skin. Take note that there are some people who have died of allergies from hair colorants in the past so this should not be taken lightly.
The first step of the test is to wash a quarter-size area behind the ear or inside of the elbow with soap and water and then dry the area. Mix and apply the hair color solution to the area with a cotton swab, leave the solution on the skin, and then check the area for any signs of irritation or redness after 24 to 48 hours. If there is no allergic reaction, proceed with the hair color service or strand test.