I just made vanilla SMBC. OMG! It is delicious! It was not as difficult to make as I thought it would be. I have seen pictures of this used to frost cakes and the cakes look very white. I was very surprised that mine turned out almost a light yellow color. How do people get it to look really white with all the butter and regular vanilla in it?
Mine is always off white. That's just the way it is. If you have violet coloring on hand you can add a dab to the buttercream. I have never done it but I've heard it cancels out the yellow color in the icing.
zinger60 - you can buy white food colouring I LOVE IT. It almost looks like you're adding paint to the mix, but it works perfectly. The one I use is 'Wilton Icing Colour, White-White'. It's a 60ml bottle I found at my local cake decorating shop for $7.00. Depending on where you live, you can probably order from the online Wilton shop also. Hope that helps!
Also, you can substitute the butter for vegetable shortening (it's called Copha here in Australia). It's 100% clear fat, so as soon as you add the icing sugar it goes bright white....much like the colour of royal icing. It's not exactly buttercream and has a stiffer consistency, but it's an option. You can also buy clear vanilla as opposed to brown.