I highlighted my hair furiously for years, and battled with it going brassy, as I preferred a cooler blonde than a warmer one.
This was by far the best toning shampoo I found. You don't use it every wash, only once a week, and it helps to maintain the shade of blonde you want.
Don't leave it on for too long, or your hair will be TOO cool and possibly purple, and don't use it too often as it can strip and dry out your hair, but if used correctly, it's such a gem!