I haven't had much luck with the wilton "white" stuff that you add to buttercream. You have to add an awful lot of it and then it left a funny taste.
My suggestion would be a whipped egg white type icing. I think it is called fluffly frosting in the women's weekly cake book (if you have that). Basically you boil sugar and water and then pour that (while hot) into whipped egg whites and keep beating for 10 minutes. It is great! I use it all the time as it is pure white and tastes like marshmallow. You might want to have a trial go first though...

