I am making a boob cake for an african-ameican. I have used quite a bit of brown coloring, but it is nowhere near dark enough. Any suggestions?

Let it sit. Dark colors like that take time to 'develop'. They should be made at least the night before using.
Also exposure to air will darken it about 1 or 2 shades.
Add some UNsweetened cocoa powder &/OR melted candy melts.
Any of that should help.