Welcome to the Cake Decorators Q&A
Hi Alicain
Begin with using the lightest colour butter you can find. Most people recommend Lurpak as it is much paler than other brands. However, it is quite a bit more expensive unless you look out for special offers. Beat the butter with a paddle beater until it becomes very pale and fluffy. Avoid using the whisk because whisking incorporates too much air into the buttercream making it difficult for smooth covering.
Adding a tiny dot of purple will give the buttercream a whiter appearance. You’d need to be really careful as even a touch too much could achieve the opposite effect. Take a peek at the following link for tutorial:
https://cakesdecor.com/entries/1338-how-to-make-your-buttercream-ultra-white
I don’t recommend using Trex as it will give a greasy mouth feel and tastes pretty awful.
Adding food whitener can also help. There are several brands available which you can see by doing a quick Google search.
Hope these suggestions help.
