They're great suggestions, thanks
Having looked at hte
GW official picture, I can see I've screwed up a little in that part of the crystal is actually just a rock, though perhaps if I paint that in either orange or ochre, the rock will fit in nicely with the desert base and gold on the model.
Deep purple crystals could provide the transition from black to light blue quite nicely, thank you both very much, I was stumped before now - I'll give it a whirl tonight
Automatically Appended Next Post: Edit - I could redo the crystals on his helm, weapon and gorget in purple too, to introduce the colour to the rest of the model.
Thanks guys, this has been really helpful