Can you color Chocolate buttercream? I have just never tried it... I assume the chocolate brown color would make any added colors look "muddy"?

You can color it black!

heheh i figured as much

You are right that any coloring added to light choco b'cream would become muddy looking. You can change the shade of the brown - play around with some extra icing and see. Take a tablespoon and add a drop of green; another w/red, orange or pink etc to see how it differes.

Maybe if you added white chocolate to it you could color it a bit? I colored some fondant with cocoa powder to a nice deep red....

If you add enough green you can make a deep moss color.
I added black and it actually kind of turned a metallic silver--why I don't know.

you might try taking tablespoons of icing and just mixing various colors to see what you get.