I'm not sure why but when I tried violet food coloring it became a gray blue color. Even when I tried to use red and blue coloring I got the same result. Does anyone have any solutions? I have a graduation cake that I need it for.

TIA,
Chele

Purple is hard to get and is not a very stable color, it will fade easily.

it takes a lot of color, so you probably have to add some softed powdered sugar and/or cornstarch to make up for the added moisture.

then, it must be protected from the light. You can help by brushing color dust on the exterior.

Sometimes it's easier to just buy a color like that, especially if you want a deep color.

http://globalsugarart.com/product.php?id=19934

If you don't want to order online, some local gourmet kitchen stores stock the bright colors, as will a local cake/candy store.