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You can’t just buy a dye and put it on your hair; it won’t work. You need to mix it with developer first to activate it. It comes in 10, 20, 30, and 40 units of volume (vol.), depending on the strength you want and damage your hair can sustain. If you’re dyeing your hair darker, you can go with 10 vol. If you’re dyeing it lighter, you should go with 20 vol. If you’re doing an extreme lightening, you can go with 30 vol., but I would NOT use 30 vol. unless you’re very experienced at dyeing your hair and know dye’s effect on it.