Jack believes that the reason he hasn't killed a pig is because they can see him coming. He is sure they cannot smell him, but he says they see "something pink among the trees". They know that color does not belong, and so they are alerted to his presence. So, in Chapter 4, Jack paints his face for the first time. It is described in ritualistic terms:
He made one cheek and one eye-socket white, then he rubbed red over the other half of his face and slashed a black bar of charcoal across from right ear to left jaw.
He now has a mask which is “a thing of its own, behind which [he] hid, liberated from shame and self-consciousness.” He can hide not only as he hunts, but as he lives among the other boys. The other hunters follow his lead, and soon their humanity is covered under their new masks. This symbolizes the end of their connection with Ralph, Piggy, and Simon, and their entrance into the world of the Lord of the Flies. In fact, it is in this chapter when Jack kills his first pig, and he proudly declares that he slit its throat himself. The red half of his face now represents blood and death.