DON'T try mixing Wilton's white and black. You'll get a purple. Their black seems to have a lot of red in it.
Thankfully the purple turned out ok for the cake I was doing it for, but it could've been a disaster!
Grey is not as simple as it seems
Wiltons Black - gives you a purplish grey
Americolor - gives you a greenish grey
Try add little bits of YELLOW - you will fidn you get a great GREY pretty quickly
I have had pretty good success mixing the Satin Ice Black (I keep a small tub on hand) with white fondant but I don't use Wilton white so not sure this was helpful.
Add yellow to which? Wilton Black or Americolor, or both?
You need to add whichever color is missing. To neutralize a purple color (red plus blue) add yellow. To neutralize a greenish color (yellow plus blue) add red.
ALL "blacks" will have a non-black tone to them once they are lightened.
This helps to explain it