Grr. I accidently used regular vanilla in my buttercream, so I tried to add some of the icing white-white color, which is supposed to whiten buttercream. The amount I added didn't do much at all. How much do you have to use to see a difference?

white white will lighten a shade of colour icing, i dont think it will make off white icing white, you could try adding just the tip of a toothpick of violet or blue to make white icing look whiter.