As I said just a few posts above, I'm in the same situation as you, Rivest and Raze_Endbringer. I'm having Kobold Brown, Ink Blue and Deepwood Green.
My guess is that the color range between these three and Soot Black is so close that the system picks one at random.

So, I would keep feeding only one fruit until the feather proc (corresponding to your current color, obviously) and pray to see Soot Black happen.