Obviously applicable to other black highlighting!

Basecoat

1. Spray Chaos Black . Pros like Darren Latham give the whole model a quick go over with Abaddon Black , but because I end up tidying the black after the gold trim, I do this at a later stage. If you're not doing trim, this stage is important to get the same finish as spray and base paints are slightly different ( Abaddon Black is more matte in my experience)

Layers

1. Start with a thick edge highlight of Incubi Darkness . Keep the paint thinned, but this is quite a chunky edge highlight. I tidy up with watered down Abaddon Black as I find this softens/feathers the edge of the Incubi Darkness layer.
2. Next is Thunderhawk Blue . You're working inside the previous layer. Again, thinned paint is important and I often use Lahmian Medium to help keep it consistent as water tends to evaporate quicker in my experience. Sometimes it's worth adding a couple of coats around raised points to build up the highlight.
3. Next is Russ Grey , You're starting to get very fine edges now and on some points of the model you may not wish to apply this - think about where the light is hitting the model.
4. For troops, I would stop at this stage, of Fenrisian Grey . These are for the highest edges and towards the tops of panels and parts of the model. If this is troop, you can also do small dots where lines intersect.
5. One step further for more important models, is to add dots and high points of Blue Horror .
6. To even the finish, apply a layer of either Lahmian Medium or, what I prefer, Contrast Medium . I add a tiny dot of Terradon Turquoise to the Contrast Medium which pulls together all of the colours in the highlight.
7. You can do light spots on circular panels like the shoulder pads or back packs by starting with a wide circle of very watery Incubi Darkness , pulling the paint to the centre of the circle. Repeat, then move on to the next colour. You will need to do a couple of layers of each colour to ensure it has the level of pigment you need.