What precautions should I take if my hair is bleached, highlighted, or very porous?

The product information explicitly cautions that on "porous, bleached, very light, or heavily processed" hair the final shade "can develop unevenly or darker than expected." Bleached and highlighted hair has an open cuticle structure that grabs pigment aggressively, so the toning shampoo may deposit more colour than intended, creating blotchy or overly dark patches. The same risk applies to hair that has been permed, relaxed, or damaged by heat styling.

Before full application, perform a strand test on a small section from the most processed area. Leave the shampoo on for the minimum recommended time, rinse, and check the result. If the tone is too dark or uneven, you can dilute the product by mixing it with a regular shampoo or reduce the contact time. Also do a patch test on skin (e.g. behind the ear) 48 hours before use if you have a sensitive scalp or a history of reactions to hair dyes. If irritation occurs, do not use the product.