What does jack paint his face like a mask? What does the mask do for him?

He wants it to hunt on the surface of things. It really allows him to separate himself from civilization and his old identity.

Jack masks his face with coloured clays and black charcoal markings, in an attempt to camouflage himself from the pigs he was hunting, but the paint which masked his face had the added effect. The paint, acting like a mask, freed him from any sense of responsibility, guilt, shame or remorse he had left. It promoted his savagery.