Is box dying hair before professionally dying it a bad idea?

Yes it actually is, you can ask any professional they'll all tell you the same thing Never box dye your hair!

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Yes, asking a professional stylist is a good idea. I am a licensed and professional hair stylist and boxed hair dye from the grocery/drug store gives inconsistent results, it fades brassy, cannot be mixed with other colors, looks unnatural, is very damaging, and is the bottom of the barrel when it comes to hair color. If you must color your hair at home, please at least go to Sally Beauty and pick up Clairol Premium Color with a bottle of developer. It's the next best thing to having your hair professionally colored at a salon.