I _think_ that's layered, starting with some middle grey (perhaps Celestra grey) and then layered with some light ash grey (Ulthuan grey), shaded carefully with a light flesh or sepia shade and then highlighted with light grey again. In the end It was highlighted with white (White Scar for example)
EDIT: Might even be layered with grey color and white mixed together, in each step you mix more white in and you use multiple very thin coats.
The step I am not sure about is the shade. There is a blueish tint that might stem from a glaze or light blue wash.... The skin might not be shaded at all in fact! If you are a pro in layering you can make that.
The scars were probably done using a purple shade and white highlights.