Wilton makes a white icing color that can be added to buttercream to make it whiter. It doesn't add any real volume to it, just changes the color a bit.
Also, you can add a very small amount of violet icing color to buttercream to make it look whiter.
There's a product called Brite White icing base. Here's a description of one that you can buy through www.nationalflavors.com:
Snow White Base -
America's finest base to make Holland cream or fluffy icing for filling pastries or cake work. High emulsification enables 15-20% more aeration, resulting in greater volume and smoother taste.
Here's cakemanOH's Brite White b/c recipe:
http://www.cakecentral.com/cake_recipe-2375-Brite-White-Buttercream-Icing.html
Duplicate post, please also see:
http://forum.cakecentral.com/cake-decorating-ftopict-591923-.html
HTH
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