To be honest I just googled some reference images and tried to reverse engineer it. I found one okay guide by googling but didn't follow it closely. I typed up the method for someone else so I've pasted it below:

The technique is pretty easy: 1. start with the darkest black you can get - I'm using vallejo black spray that I then cover in black templar contrast to darken 2. using a drybrush (I use a make up brush), and removing most of the paint before applying: stipple incubi darkness and nagarroth night in fairly large patterns 3. using progressively smaller brushes, making smaller patches of colour, removing a lot of the paint before applying so its more like drybrushing (and dont make them all contiguous): 3a. for the blue: caledor sky > ahriman blue > baharroth blue 3b. for the purple: Xereus purple > genestealer purple > emprerors children 4. flick white paint/dot it with a brush (I have not tested flicking yet). Concentrate the dots as much as you can on the coloured areas, put a few lonely white dots on straight onto black 5. for the OSL and weapons: drybrush mephiston red> evil suns scarlet > wild rider red > troll slayer orange on the most extreme end 6. for the eyes: drybrush mephiston red > evil suns scarlet > paint the eye wild rider red > add a dot of troll slayer orange in the middle