I want to make a cake that looks like a tombstone, how can I make the white frosting gray? Please help?!

I want to make a cake that looks like a tombstone, how can I make the white frosting gray? Please help?

My friend is having a "RIP Summer" back-to-school party. Like with black and gray balloons and stuff. It's gonna be cool. Anyway, I'm in charge of the cake and I want to make a tombstone/gravestone/what ever you want to call it that says "RIP Summer." What would I use to make the white frosting gray?

14 hours ago
I didn't knwo they made black food coloring....that was a waste of 5 points...oops :)

If you only have the standard liquid food coloring, use a combination of blue food coloring with a little red and a touch of yellow. Experiment with a small amount of frosting until you get the right amount of each. The color would be mostly blue with just a touch of the other two.

The professional type paste colorings are available in black from craft and restaurant supply houses. You would then use just a little bit of black.