Multiple Processes
Exposing your hair to multiple chemical processes back-to-back can be damaging to its health. It is particularly straining to first darken your hair and then highlight segments of it through bleaching. If possible,you should never do more than one at-home process. If you dye your whole head with a rinse and then immediately add highlights to just treated hair it can cause a number of adverse chemical reactions. As a result, the highlights may be a different color than intended, your hair may become brittle and damaged or worse it could break off entirely and fall out.
Salon Hair Care
If multiple processes are needed to achieve a desired look, the optimal choice would be a salon. An experienced colorist will know which dyes can be safely combined, to avoid permanently damaging your hair. The colorist may also suggest that you wait a period of time in between treatments; or prescribe a color-safe conditioner to use on your hair. Hair that is frequently chemically treated needs to be trimmed regularly to eliminate breakage and split-ends.
At Home
While not the ideal choice, there are safe ways to do multiple hair processes at home. A new product called Revlon Custom Effects is an all in one kit that simultaneously changes hair color and adds highlights.There are also two-step dye and highlight treatments like Couleur Experte by L'Oreal. These products are designed to give hair a perfectly matched, multi-tonal look without over-processing or damaging your locks.