Huh, I'm not quite sure. I know that Wilton makes a "white" type stuff that will help make things white again! I can't think of what it's called off the top of my head. I saw it at Michaels when I was there! I hope this helps. If nothing else, it'll give you a bump! 
Add a tiny bit of violet (a drop at a time or a little bit on a toothpick of the gel) and mix until it appears white. I know it sounds odd, but violet cancels out yellow on the color spectrum, and your icing will get whiter. Just add a little at a time until you get the white you want. Too much violet will obviously start making it violet.
I do this all the time, especially when I'm making a wedding cake. I use butter in my icing, and it yellows it a bit. The violet trick really works!
One note about it though -- sometimes, I've noticed that the icing takes on an oh so slighly gray appearance in the KA. Don't worry, it will still look white when spread on. I actually practiced this the first time by icing a cake with my regular icing and then using the violet and icing over the original stuff. It was Waaaaaaay whiter!
Good luck!
S.
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