Jack first talks about painting faces in chapter three, excitedly talking about how they can paint their faces and then sneak up on the pigs while they are sleeping.
In the next chapter, Jack is working on his mask for quite some time as he wants to make it just right. Because once he covers his face, he feels a complete escape from responsibility and the rules of civilization. Golding writes that "Jack hid, liberated from shame and self-consciousness." This change is an important one as Jack is suddenly allowed to overcome his natural inhibition and can now kill pigs.
This is just the first step down the road that will allow Jack and the others who follow him to use violence more freely and eventually murder both Simon and Piggy. Recalling Jack's inability to stab a pig at his first encounter, the painted face is an important step down the road to savagery.