I have been getting my hair done professionally for quite some time now. I get platinum blonde foils but that has been getting far too expensive so I decided to by a lifter and do it myself. The over all color came out great apart from the roots where my natuarl color was showing, they turned out a little brassy. I used a purple shampoo which has helped a little but how can I make the brassyness go away completely and prevent it from happening next time I need to touch up my roots?

You didn't say what you used, and that is important. Basically, you need more "lift" to get pass the brassiness. Here are some *possible* ways to get there.

1. Use heat to accelerate the lightening action
2. Increase the time the product sits on the roots
If you're using single-process color:
3. Use stronger developer
4. Bleach a few highlights
5. Use a product that contains at least some bleach, or use all bleach.

I already asked the following question:

"I have been getting my hair done professionally for quite some time now. I get platinum blonde foils but that has been getting far too expensive so I decided to by a lifter and do it my self. The over all color came out great apart from the roots where my natuarl color was showing, they turned out a little brassy. I used a purple shampoo which has helped a little but how can I make the brassyness go away completly and prevent it from happening next time I need to touch up my roots?"

Thank you Perito for the informative answer you provided. I was thinking maybe I could just do the roots again and just leave it on there for longer. So the coloring crème I used was an 11.0 by hair sm and mixed that in with a 30vol developer. Is it recommended I didn't it again and leave it on for longer? Or is there something else that I should be doing?

No. That sounds about right. I recommend that single-process colors be left on for a full 45 minutes. After that, the product is largely consumed. 30-volume will lighten it a bit more. There are other options if that doesn't quite do the job (40-volume, double 30-volume, etc).