I had to make blood red gumpaste roses for a cake and I bought the stuff Earlene Moore
sells. Her directions are to replace the water in your gumpaste with this color paste. It did give a very deep red without that horrid pink. I used it as is for the first petals, then lightened it a bit with white gumpaste for the next set of petals, then lightened some more, etc. for color gradation. I don't know how this paste would work in buttercream, but it worked great for gumpaste.
You can't get dark colors with the natural food coloring. Beet juice gives a nice natural looking pink in buttercream and if you try to add more, it'll just break down the buttercream and/or it will taste like beets by the time you add enough.