It may sound like a bad infomercial but believe it or not, I hear this ALL the time! Many of my blonde clients come to me with this issue.
There are two main things at play with "brassy hair", one of them is your current stylist is not bringing your color to it's lightest point without damage. Or, you may be going through a color correction, in this case you need to be patient in that your hair can only take so much change at one time. You may need additional sessions to get were you want to be.
You have to also understand that hard water is not the best for your hair. It can cause ashy and dull, green colored hair due to the nature of this water, especially if you have light colored hair. More to come on this subject in a future blog post.