I wonder if it's just the brand, or if it's old... What are you using for your buttercream, and white kind of coloring are you using?
I am having a problem with gray gumpast that is supposed to be black. I have decided to "paint" the bow loops to get them the right black color.
I know if you use butter flavored Crisco in your buttercream, it can really make the color not what you want.
I sometimes use chocolate buttercream, add black coloring, to get black icing.
It might be because you icing has a slightly yellow tint, to fix that I would try adding a little bit of purple which will conteract the yellow, then try adding the black again.
Deborah
Are you using real/imitation (brown) vanilla? Sometimes that has an affect on the coloring as well.
Use clear imitation vanilla if you can get it and you shouldn't encounter that problem.
The best colors I have found to use are the Americolor gels. I always get true colors when using this brand.
The final thought is that the color is just old.
A little bit of black should turn your white icing gray not green 
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Hope this was a little bit of help.
Sorry if it wasn't. Good luck though! 
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