I have someone who wants buttercream icing that is a deep dark brown. She doesn't like chocolate so I have to use colors only. Any advise would be appreciated. TIA

Maybe this is too obvious but use brown gel coloring and a bit of black.

I'm using brown and black gel colors but the brown isn't turning out dark enough. The woman wants the color to be the color of DARK chocolate. I'm afraid if I keep adding more and more color to the icing it's going to turn out bitter. Does anyone have a special technique to acheive a really, really dark brown. TIA

The Americolor ones work better than the Wilton gels, if you can get them.

Dark chocolate is pretty dark, I'm not sure you'll achieve that without using some chocolate icing recipe first.

Are you covering the entire cake with it?

If your icing has a reddish tint to it, add some green. It will darken it, and give it a more chocolatey color.