I have a sensitive scalp and previously coloured hair — what precautions should I take?

Because this is an oxidative permanent dye (it uses an alkaline agent and hydrogen peroxide to open the cuticle and develop colour), the scalp is exposed to potential irritants like ammonia substitutes, resorcinol, and p-phenylenediamine derivatives. The manufacturer explicitly advises an allergy alert test 48 hours before each use — even if you have used the brand before — because sensitisation can develop at any time. Apply a small amount behind the ear or on the inner forearm, leave uncovered, and watch for redness, itching, or swelling. If your scalp is already sensitive, protect it by applying a thin layer of petroleum jelly or a dedicated scalp barrier cream along the hairline and partings before mixing the colour. For previously heavily processed lengths (bleached, permed, or repeatedly coloured), the cuticle is more open and will grab pigment faster and darker. To avoid overly dark, brittle ends, you can either apply the mixture to roots first and pull through only the last 5–10 minutes, or dilute the developer for the lengths with a conditioner (1:1) if the instructions allow. Always respect the maximum development time printed in the leaflet; extending it will not deepen the shade but will increase damage and irritation risk.