Well i use two methods depending on how big a character they are or how lazy i am
Lazy:
paint boltgun metal, wash with badab black, wash with purple ink or wash (i still have some of the old inks which i prefer on this occasion), then dry brush mithril silver. Quick and easy
Wolf Lord/Character:
Undercoat black, use many many thin layers of Liche purple, fading from purple at the tip to black at the base of the claw, then a final highlight of 50/50 tentacle pink/liche purple along the edges. sometimes i wash over this with watered down purple wash to blend the highlight in a bit. Takes a while but looks good (i think anyway)
My Spacewolves are mainly grey and red so i use purple on any power weapons/claws/frost blades just to make them stand out a bit. Could look good on your Flesh Tearers too?
Hope that helps
Rev