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Go to a salon or get actual color and developer from a beauty shop such as Sally's, no box dyes.

Your hair will look much darker when you first color it by yourself because box dyes aren't accurate and it will be such a big change. If you were to get a box dye that said "dark brown", it would come out looking black. You'll have much better luck going to a salon than risking messing your hair up and spending tons of money to fix it. If you have hair schools around you, you can normally get your hair colored there by students with professional supervision for cheaper as well.