The mask created by painting the face serves the main purpose of allowing Jack, and later, the other boys, to let the inner beast out. The mask is something for the boys to literally and figuratively hid behind. The civilized personality can be hidden behind the face paint just like a person's true looks can be hidden behind a plastic mask. When Jack paints his face, he feels free to let out his inner beast. The mask also removes a degree of humanity from Jack and the rest of the boys. Humanity, or civility, is what keeps the beast inside of most people, according to Golding. Society has rules that most people learn as they go through life, this makes the people more human and less like animals. The face paint removed that somewhat. In chapter 4, when Jack gives a command after painting his face, the narration tells us that the twins responded because "The mask compelled them." The mask also affords just enough anonymity to help relieve the wearer of responsibility. If no one knows who to blame, then no one has to take responsibility for actions. The mask gives that degree of freedom to the wearer, thus again, making it easier for the evil inside to come out.
In most literary works, masks function as a concealment of one's true identity, both literally and figuratively. In the case of the masks/painting/camo, Jack becomes separated from himself, and is freed from accountability by becoming a "new person." The masks allow the boys to function within the realm of a new identity, and as they don their new personas, they are led to act in ways that are primal at best, and atrocious at worst.
Beyond Lord of the Flies, other authors have also used masks for symbolic reasons -- think of the masquerade ball in Romeo and Juliet, for instance. Other authors have also used masks in their literary works for similar reasons. A quick Google search of "masks in literature" should guide you to more information.
Jack enjoys painting the mask on his own face, and looks at himself in delight:
He looked in astonishment, no longer at himself but at an awesome stranger. He spilt the water and leapt to his feet, laughing excitedly. Beside the pool his sinewy body held up a mask that drew their eyes and appalled them.
First of all, the mask makes Jack a stranger, it liberates him from his sense of self. It makes him excited - and better, the mask draws people's attention and makes them slightly scared and repulsed. It gives Jack power.
He began to dance and his laughter became a bloodthirsty snarling. He capered toward Bill, and the mask was a thing on its own, behind which Jack hid, liberated from shame and self-consciousness. The face of red and white and black swung through the air and jigged toward Bill. Bill started up laughing; then suddenly he fell silent and blundered away through the bushes.
The mask separates Jack from a sense of autonomy, makes it seem as if it is not Jack but the mask, "a thing on its own". The mask, as well, Golding is clear to point out, liberates Jack from "shame and self-conscoiusness": Jack feels like he can get away from any sense of responsibility.
The other thing to say about the mask is that it makes the boys seem less "British", and far more "savage": like a painted tribe, it returns the boys to a more primal appearance.
It is, undoubtedly, a key factor in allowing the boys to commit the atrocities they commit.
Here is that scene in the film adaptation:
The mask provides anonymity. And anonymity is crucial to immoral behaviour, especially for socially weak people. Behind anonymity, you can judge without being judged, you can act unpleasantly without repercussions. You can unleash the angry inner-child that is bottled up inside you. Without the mask you must be polite. But when anonymous, you can trash everyone-else and pretend you are some kind of superman. It stems from deep insecurity.
This is what we learn from William's work. Golding was very astute.
- Jack paints his face to unleash himself from civilization; he found that the mask was able to liberate his real identity. “He capered towards Bill and the mask was a thing on its own, behind which Jack hid, liberated from shame and self-consciousness.”(Golding, pg.66). While Jack still feels the influence from his civilized life in the past, the mask is able to hide his real identity, and help release evil and savagery. Connection with the war is also present; Jack’s camouflage mask was used by soldiers in the war, where they fought shamelessly and killed behind the camouflaged masks.