Question about dying hair a cooler shade of blonde

Hi,
  So I recently dyed my hair blonde and I'm not completely happy with it. It was previously dyed a reddish/coppery brown (natural hair color is a light brown) so my colorist bleached it, put a light ashy blonde color over it, and toned it. The problem is it's still a bit too yellow for me. I have a box of L'oreal Dream Blonde in the extra light blonde color and was thinking of putting this color over my hair in a couple of weeks as the l'oreal color looks more like the platinum/white blonde I'm going for and I don't want to spend any more money at the salon. I was just wondering, if I put this  cooler shade of blonde over my warm, yellowish shade, will it turn out basically like the color on the box or at least closer to what I want? I just don't want to have another color disaster, something I always seem to have when experimenting with blonde shades. Anyway, thanks for any advice you can give =)

I agree,try a strand test first.If the hair is the color of the outside of a banana,it will be fine.If it is more like the color of inside of a banana,it
may pull slightly greenish,but if that happens,it usually washes out in a few washings.