Well, many people asked me how to paint this tattoo pattern. It actually quite easy. You need to have a very sharp tip Kolinsky Saber brush. I came up with the step-by-step for 3 types of patterns following GW.
Here's another commissioned work for my Australian client sometimes ago. He wanted me to work on army that can be usable in Eldar as Dire Avenger/Guardians and in Dark Eldar as normal warrior. And more 'pirates' type. So the best way was to use just Dark Eldar warrior......
Choosing color schemes.... He selected this from the Dark Eldar codex: