Thanks 
Try and and add some cocoa powder. If you get the Dutch process cocoa it is very dark brown, the regular cocoa is a bit more brick coloured.
Add some dark cocoa powder (100% cocoa, not cocoa drink which has some milk solids and sugar in it) . It turns a nice dark brown and gives a yummy chocolate taste.
You also may need to reduce sugar in your buttercream and add melted dark chocolate to it to balance the taste and texture.
I went through that whole dark brown chocolate thing myself. I kept adding brown food coloring, wanting the dark color with fresh BC dark, not realizing that it turns much darker as it crusts up. Just adding a bag of melted semi-sweets gives me a dark icing without food coloring. But you wouldn't think so by looking at it right away.
When I make chocolate buttercream, I use my regular recipe, then add as much cocoa as I need to get whatever color I want. If you're going for a dark chocolate it may take 1/2 the container of cocoa or more depending on how much frosting you have. You will have to add more milk or water to your frosting to compensate.
I ran into the same problem once and I needed darker brown fast, brown coloring didn't work and a run to the store was not an option. I had some store bought chocolate fudge frosting on hand, I added that to my homemade chocolate buttercream and it worked great. HTH 
Hershey's Special Dark cocoa powder does it every time!
No such thing as too much chocolate! I say just go for it! If you use dry cocoa I would sift it together with a little powdered sugar to stop it from clumping, then add it to your BC with a little more liquid until you get the right consistency.
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