A buttercream frosting made with mostly or all shortening and little or no butter will be whiter than frosting made with a more standard combination of the two. But if you want it even brighter white, there is a little known trick you can use—but proceed with caution! Add a tiny drop of blue food coloring in the frosting, which will neutralize the yellow cast. But truly, start by dipping a toothpick in the food coloring and letting a tiny drip fall into your frosting. At first, it will look blue, but keep mixing, and the tint should disappear, leaving you with bright, bright white buttercream.
Recipe: Vanilla Buttercream Frosting