Why do they face paint?
From a study of Apocalypse Now:
Masks are used at key points throughout the film to symbolize the anti-self—the new identity each character assumes in order to deal with the war, an act that requires a symbolic killing of the old self. Willard’s smashing of his reflection in the first scene suggests such an act of self-destruction. By the end of the movie, numerous characters have donned masks or painted their faces with camouflage, signs that they are no longer themselves. When Lance seems finally to reach his breaking point, he drops acid and hides his face in camouflage paint. Kurtz’s face is often obscured by shadow or darkness, and when Kurtz throws Chef’s severed head into Willard’s bamboo cage, he does so wearing face paint. Finally, when Willard prepares to kill Kurtz, he covers his face in mud. These masks underscore the dramatic transformation of the human self during wartime.