The method I used to paint most of my skulls, horns, teeth and claws. High contrast against black, very simple to do.

Basecoat

1. Use Ulthuan Grey to basecoat the areas needed. It can be chalky, so thin coats, and make sure it's dry. Often needs a few coats!

Shading v1

1. Wash all over with a thinned Nuln Oil . I often add just a small amount of Nuln Oil Gloss to provide a more satin finish. This has to be very thin.
2. Push a bit more of the above mix into recesses.

Shading v2

1. Alternatively, use Apothecary White all over
2. Add a bit of Black Templar to the previous mix and push into the recesses. Worth adding some Contrast Medium just to give more control

Highlight

1. Go back with thinned Ulthuan Grey over edges, teeth, tips etc.

Optional Highlight

1. Use White Scar to add dots of light to highest points, teeth etc.