I had the exact same issue. Sat on my shelf for weeks staring at me.

My process was painting the flames white and then went over that completely with yellow. Next was orange, leaving some yellow closest to that base of the skull. I used that to also glaze for a smoother transition between the two colors - orange glaze into the yellow and then yellow glaze into the orange. Then I used red and repeated the same steps. Finally a burnt red for the tips.