Haven't used contrast myself, but did look at them in the store and they walked me through how they work a bit.
It would work nicely for wyches. As Alino says, the flesh would work. As would the hair.
For the clothes, it depends on the colour you're going for. If you're going to do black, then you'll be using the regular Abaddon Black anyway. (I don't think there is a contrast paint for black, but could be wrong).
For the armour, it will also work nicely depending on the colour you're going for. I personally have always done mine with painting them Eschin grey and then several washes of Coelia greenshade. Using contrast would be much the same.