Using goldwell on brassy roots?

Sorry, I'm not a pro and don't know about the ash tint mix. To counter orange you'd use something with a blue base, to counter red, a green base, and to counter yellow, you'd use a blonde with a violet base. The base is written on the dye, but all of those bases tend to come in colors called "ash" as opposed to "golden". The color itself isn't going to lift medium brown though. Even if you use a 40 volume developer (the highest peroxide), with a high lift shade, you still may not get enough lift, so I'd definitely test strand it.

If you really want platinum whitish hair, hair bleach, then violet toner is probably going to be the route that gives you the healthiest hair (repeated 40 volume use can be very damaging).