In addition to the quotes given in the other answers, here are three more.
When Jack has left Ralph's group to form his own tribe, he begins to wear face paint as a sign of his authority. When Roger comes up to Castle Rock after passing the checkpoint:
The chief was sitting there, naked to the waist, his face blocked out in white and red. ... The newly beaten and untied Wilfred was sniffing noisily in the background.
Face paint, or the mask, allows Jack to perform a violent act against Wilfred that does not need to be explained to the boys. Roger recognizes it as "the possibility of irresponsible authority."
Later Jack informs his tribe they will hunt again and have a feast, but one of the boys asks how they will get fire.
The chief's blush was hidden by the white and red clay.
Jack's light complexion predisposes him to blushing, which gives away his insecurities. The mask allows him to hide his emotions that are too often betrayed by his normal skin shade. Thus the mask allows him to don a facade of self-confidence.
In chapter 11, Ralph staunchly refuses to give in to the temptation to wear a mask when he goes to confront Jack.
"But they'll be painted! You know how it is."
The others nodded. They understood only too well the liberation into savagery that the concealing paint brought.
"Well, we won't be painted," said Ralph, "because we aren't savages." ... Ralph shouted. "No paint!"
Here Golding clearly states the link between the masks and savagery. By concealing themselves behind paint, Jack's tribe found it easier to forsake their morality. Refusing to wear a mask, Ralph holds on to the standards of civilization. "We must go as we are," he says. They have lost the grooming and clothing that had identified them as civilized beings in the past, yet Ralph is not willing to forfeit his upbringing and his sense of right and wrong. By not allowing his group to hide behind masks, he asserts his commitment to civilization and morality.