BRASSY HAIR - PLEASE HELP?

I went to Sally's Beauty Supply and purchased "ion Color Brilliance" hair color to cover my brassy/yellow highlights. I was told to use 10 volume developer and mix with the hair color (shade: 7A-7.1) similar to my natural dirty blonde. I mixed it up very well and applied to my hair for 10 minutes. I washed my hair and I see no difference. My hair is still the same. What went wrong? I also used purple shampoo many times before and it never helped either!