Buy a light ash brown, it happened to me once, i didnt like my blonde high lights and i bougt this color and it coverd them and i had a nice color

This is why you must do your homework before letting anyone mess with your hair. When I was looking or a new place, I walked in during an off time (early morning) and asked the person at the front desk who was the best colorist for natural looking blonde highlights, and then I chatted with the guy for a while asking him questions about his technique. He knew his stuff. Fwiw, I prefer stylists trained in Europe who avoid using foil. Do not fix it yourself. Shampoo it and heavily condition it, and go back and ask them to tone it down.