Is a sensitivity test really necessary every time, and how do I do a strand test correctly?

Even if you have used this exact shade before, formulations can change and your immune system can develop new sensitivities. Mix a pea-sized amount of colorant with the same ratio of developer, dab it behind the ear or on the inner forearm, leave it uncovered for 48 hours, and watch for redness, itching, swelling, or burning. If any reaction occurs, do not use the product. For the strand test, take a 5 mm wide slice of hair from the nape or behind the ear — an area that matches your overall porosity. Apply the mixed color using the same timing you plan for the full head (usually 30 minutes for standard coverage, up to 40 minutes for resistant gray). Rinse, dry, and evaluate both the shade and the hair’s elasticity. If the strand snaps easily or feels gummy, the hair is too compromised for a full application; consider a protein treatment and wait two weeks before re-testing. The strand test also reveals whether the ash tone neutralizes your underlying pigment as expected or whether you need a different shade or a pre-toning step.