It is better to use overtone on dirty hair. Although overtone is not 100% dye, however, since it deposits color into the hair just like dye, the general rule for dye application is also applicable. It is often advised that dyes are applied to dirty hair because of the healthy benefits associated with it. Dyeing your dirty hair will help preserve natural hair which would have been removed by shampooing. This natural oil protects the hair from the effect of exposing them to harsh chemicals. Apart from that, they also serve as extra coats which protect the hair from breakage, damage, and irritation of the scalp when the hair cuticles are broken down during the application process. The point is, that the more natural oil that is present in your hair, the more protection you get during overtone or dye application.