Blue Highlights

Tools and products to highlight hair
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Q: I have my hair colored blue black and now I want to do highlights and foil a blue color. Can I do blue highlights with foil on blue black without bleaching first, or must I bleach my hair and then do blue highlights on blue black? Which peroxide is better, 20v or 30v?
A: Well, if you want more than simply a rich blue cast to the hair when the light shines on it, you will have to lighten the color of the highlights before you can add a notable blue tone.
You will need to use a bleaching agent mixed with 20-volume peroxide (10-volume if your hair is damaged or porous) and carefully monitor the processing as it lightens. This type of highlighting is generally easier to accomplish using a highlighting cap, if you are working alone. If you have a partner to assist, you can probably do foils with a little practice.
Be sure to carefully monitor the lightening checking the color changes at least every 5 minutes. Rinse the bleach from the hair using cool water, and gently shampoo the lightened hair before using the color to tone the lightened hair and be sure to dry the lightened hair fully. (This ensures the removal of all the bleach.) You can generally do these steps without removing the highlighting cap, which will help you avoid the need to re-isolate the lightened hair.
After the lightening, you can apply the blue hair dye and achieve a rich, vibrant blue color. Just remember to condition the hair well, after the coloring process since the hair is going to be much more porous.
As an alternative, you can remove the highlighting cap after you've lightened the hair and rinsed the bleaching agent, but before shampooing. By doing this, you can shampoo and dry the hair, then apply the blue color to the entire head to reinforce the blue-black tone of the base color, while adding the vibrant color of blue to the highlighting.
See also:
How to color hair
Foil highlighting
Highlighting with a highlighting cap