My daughter has very light blonde hair she wanted brown highlights so I bought a dark brown color and now it looks grey what should I do to fix this and turn it brown?

I'm a hairdresser by trade,I would recommend to go to a hairdresser so the problem doent get worse.. If not you need to choose a golden brown to help deposit colour... When we lighten hair depending on how many levels, you will generally see orange.. This is an under tone. You need to replace this when going darker there fore by choosing a golden colour.