The hydrogen peroxide in bleach opens the cuticle and allows the colour to go into the shaft. Cecilia Cuba says in her article, "The Chemistry of Hair Highlights," that the bleach makes hair more porous, so applying a toner right away will help the colour go into the open cuticle and last longer than if done later. If you wait to apply it, the cuticle will close and the toner may not set properly.
If Your Hair Needs a Break... Wait.
If your hair is distressed from over-processing, then wait a day or two to tone it. Applying too soon will continue to open the cuticles and eventually the hair may break. If you dislike the highlights, then maybe reconsider toning and dye while the cuticle is still open. If you believe the highlights will lighten in sun or naturally with body heat, then hold off.
If the highlights are brassy or too bright apply the toner after washing the bleach off. It is designed to glaze the bleach and make it look more natural. Doing it right away will help it last longer. However, if your are considering redyeing the hair anyway, do not tone.
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