Lighteners come in two basic types - on-the-scalp lighteners and off-the-scalp
lighteners. They're classified as such because they are designed to be safe when
applied on or off the scalp as the name indicates. Off-the-scalp lighteners are much
stronger and should never be used so that they come in direct contact with the scalp
because they will cause chemical burns. These lighteners are generally designed for
use with highlighting or frosting caps where they don't rest on the head.
On-the-scalp lighteners are formulated to be safely applied to the hair and in the
scalp area. Often, on-the-scalp lighteners are slower working, but infinitely safer for the scalp.
Using any lightener requires careful monitoring of the hair as it is being
decolorized, because it can easily be over-processed. However, once you get your hair
lightened to the desired level, you can then color it as you desire, and know that you'll get the color you are trying for.
You should also remember that while haircolor when mixed with a developer goes
inert after 30 minutes, lighteners will remain active as long as they are moist. As a
result, even a lightener mixed with only 10 Volume Peroxide can result in over-processing if not watched carefully.
You also want to be careful that your hair is in good condition before you lighten it.
Damaged or porous hair can become irreparably harmed by lightening.