How to keep hair from turning brassy???

I have my hair highlight and low lighted. The dark is really dark and the light is really light. I love it, but it's been a few weeks since it was colored and I'm noticing the blond is turning slightly brassy.

How do i keep it from doing this??

I've used sheer blonde shampoo before which helped, but I'm hesitant to use it now since the majority of my hair is DARK, which I don't want to fade. Will this affect the overall color at all?

Do the violet shampoos work??

Please help!

How to keep hair from turning brassy???

I've not done highlights, but have found that if I stick with colors that are ash based have less tendency to be brassy.

How to keep hair from turning brassy???

the violet shampoos are AWESOME my best friend is a hair dresser she gets it for me all the time there is also a shampoo for dark hair called "onyx"