As Japanese hair straightening uses a chemical process to reshape the hair, it could cause damage in coloured hair. Tomoyuki says that those with lightened hair above level five (dyed blonde and bleached hair) might not benefit from the treatment, but this goes for hair which has been dyed darker, too. "Some clients have dyed their hair dark and think their hair is healthy, but it doesn't work this way," adds Tomoyuki. During your consultation, let your stylist know if your hair has been dyed, so that they can assess the propensity for damage.