I think when I get my Chorfnought I'm going to paint all the flames in different colours. Red, blue, green, purple and such.
I really like this idea because it kind of represents the idea of all the various deamons that are chained up within it. Each colour representing a certain "essence" so to speak.

The symbol on the bull's forehead reoccurs a few times across the army and is a dead ringer from the seal of Grungni that's present on all the thunderstrike models