I read on another post (thank you to whoever posted this tip!!!) that you can add just a bit of violet to make the icing whiter and it actually works! Something about violet is opposite of yellow on the color wheel. You just have to add a tiny, tiny bit at a time. I do that until it looks pretty good then I add some of the wilton white to finish it off. The poster said if you get too much violet you can add a touch of yellow to go back the other color direction.
I too almost had a heart attack when I saw how much the natural maket butter was!