What Does Bleach Do?

Bleach for hair
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Q: I have some blonde in my hair and plan on re-coloring it. I was planning to use the same hair dye I used before and do my hair the same way, but my sister says that I should bleach the hair instead of dyeing it.
What does bleach do? Doesn't it make your hair blonde? I have a friend who used bleach and it looked really bad. I really don't want my hair to be fried. Can you explain to me the difference?

A: Bleach does indeed lighten the hair. It is generally used when the hair is desired to be a color that is significantly lighter than the starting color. As a rule of thumb, when the hair is desired to be lightened by more than three levels, you may want to use a bleaching service prior to a toning application.
In your case, if the hair color product you used before gave you the results you like, I see no reason to change things on a second go around. When color is mixed with developer into a lightening formula, that formula has a life-span which minimizes its efficacy and therefore the amount of damage it can do.
Bleach on the other hand, keeps working as long as it is moist, and can easily over-process, resulting in damaged hair if you aren't sure of how quickly your hair processes.
I generally advise leaving the bleach applications to professionals.
See also:
Bleach hair
How to color hair
Bleaching retouch and hair breakage
What is hair bleach made of? Is bleach harmful to your health?