What is the significance of the face paint>?

The significance of the face paint is pretty important. First the paint allows the boys to free themselves from their old life and identity. This is particularly important to Jack. Jack was already a socio-path before the novel started. As he smears the paint on he begins to snarl and dance around. This is the outlet he has been looking for to lose the old world. The paint also allows the boys to identify with each other. There is a sense of savage belonging and comfort if everyone has paint on; it is sort of like gang-colours. I suppose camouflage for hunting enters into it but it is secondary.

The boys also use the paint on their faces as masks to hide their shame and the guilt they feel over brutally murdering the pig.