I'm trying to get a grey buttercream. When i add the black it turns a greenish shade (almost an ugly forest green) not grey at all. I was told to add black fondust to the white buttercream but it still green. My buttercream is made with salted butter, icing sugar, cream & vanilla, so i dont know if that is whats causing it to be greenish? I have to make an elephant cake for my nephews 1st bday & im at a loss of how to do this! I didnt think it would be this hard. Ive seen the previous posts about this and tried what they suggested but it still hasnt worked. Any one have any tried & true tested ways to get this?

What brand black food coloring are you using? Black food coloring is really made up of a bunch of different colors like purple or green. So if you just use a little of it, it tends to bring out one of those colors. Wilton's brand especially comes out greenish. Americolor makes nice bold colors so I would try Americolor if you haven't already. That brand is always true to color in my experience. 

Quick research and I found black food coloring is made of red, blue, and yellow. Since yours is coming out green, try adding a little red to your frosting since yellow and blue make green. There must not be enough red in that brand you are using.

Depending on the brand I use, some are greenish and some are too purple.  (even the fondant I buy is never true black).  So if it's too green, try adding a bit of purple at a time.  If it's too purplish, add a bit of green at a time.  They are opposite on the color wheel.

Yellow cancels purple.  Red cancels green.  And vice versa.  Just google color wheel.  Opposite colors will cancel out or lighten your color.  As ‍hotpink lipgloss stated, black is the combination of the primary colors.  Personally, I use AmeriColor Super Black and get a nice gray from just a dab.

hmm it is the wilton brand, i have a few Americolors but not the black, i'll try that one out instead! And yes the Americolors are so wonderful!

I always have to tweak my color when I use a dot of black to get grey. You need to used either red or yellow depending on which direction you want to go. If it looks a little on the purple side, add yellow. If it looks a little on the green side, add red. Compare it to something white to get an idea of the direction your color is going so you can adjust as needed. I'm a fragrance chemist for Avon Products and color matching is my favorite part of my job. 

I also found that wilton makes a gray green, not a nice stone gray.  By experimenting with what I had, I found a drop of black Kromacolor from my airbrush colors was perfect.  If you have that on hand, give it a try.