Please take my advice with a pinch of salt, this is only how I would approach them:
1) prime them white
2) apply a black wash, concentrating on the recesses.
3) obviously only using thin layers I would then start applying the desired colours, starting with the darkest, i.e. start with (
GW colours) liche purple, then 50:50 liche and warlock purple, then warlock purple.
4) highlight with flesh shades (dwarf flesh etc) up to very thin white highlights.
5) Paint the mouth black
6) I would paint the teeth paint bleached bone, then a brown wash, then bleached bone again, followed by skull white highlights. (this applies to horns or claws as well)
obviously the above is for the deep purple horrors you would change your palette for the blue horrors.
I hope this helps

sorry if it doesn't