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.