As was mentioned in the previous post, Jack initially paints his face as a way to camouflage himself while he is hunting. In Chapter 4, Jack walks out of the forest holding a stick of charcoal and leaves filled with white and red clay.  Jack explains the reasoning behind painting his face to Roger by saying, "For hunting. Like in the war. You know—dazzle paint. Like things trying to look like something else—" (Golding 88). The other hunters are initially taken aback after witnessing Jack's newly painted face. When Jack looks at his reflection in the pool of water, he is also astonished. Golding writes that Jack felt as though he was looking at the reflection of a total stranger. The mask seems to take on a life of its own and liberates Jack from "shame and self-consciousness." Jack convinces the other hunters to follow his lead and paint their faces. The clay and charcoal masks allow the hunters to act like savages without feeling self-conscious about their behavior. The paint hides their identities and gives them the freedom to act like barbarians. 

Initially, Jack paints his face in order to hide himself better from the pigs he's hunting. He concludes that the pigs can't smell him, but they can clearly see him as he stalks them, and so he needs a sort of camouflage.

Later, the mask takes on added significance because it hides Jack from the other boys as well. There is a wealth of symbolism in masks as a literary and cultural element, and most of them are employed here. The mask allows Jack to "not be Jack", so to speak; it frees him of his identity and all the restraints that go with it;

He capered toward Bill, and the mask was a thing on its own, behind which Jack hid, liberated from shame and self-consciousness.

The mask also represents a descent into savagery, in that a rational "civilized" person would not have to take on a persona or employ magic in order to accomplish a task, and the things Jack is liberating himself from are all generally considered to be good traits in a civilized relationship. 

Why did Jack paint his face in Lord of the Flies?

Jack Merridew's primary goal is to be the unequivocal leader of the group of stranded boys. He is power-hungry and determined to be in control. Fearmongering is also part of Jack's strategy, which is why he plays up the story of the so-called "beast" to frighten the younger boys.

The reason Jack paints his face is linked to all of these factors. His initial motivation is to achieve better camouflage while hunting pigs after deducing that his efforts have been hampered by his inability to hide. His mask of paint, however, became a symbolic representation of his quest for power and the loss of his humanity.

Of all the children trapped on the island, Jack is arguably the one who loses his humanity the most. His face paint allows him to create a kind of barrier between who he used to be and who he is becoming. If showing one's face is considered "normal" and indicative of societal convention, then Jack's mask is a symbol of him embracing his barbaric leanings.

Jack's camouflage also provides a valuable way for him to hide from the other boys when he wants to. Being hidden keeps him in control, enables him to easily evoke fear, and keeps the power firmly in his hands.