For black add some red or purple. I don’t know what brand you are using, but most need time to develop the color. Try making your colors and letting the icing sit for a couple of hours to let it darken. The queen of food coloring is Lila loa. She has a great book where she talks about how to make colors (and other things). She also has a blog. Good luck!
@Econlady Thank you for the tips.
I will also check out Lila Loa's blog