Is a War Boy's skin white because of radiation or some other factor out of their control, or do they purposefully paint themselves white for some reason?
All Warboys have their torsos, arms, and heads completely covered in a white powder as a tribute to their revered leader Immortan Joe, with many using black grease, or pigment around their eyes, or foreheads. They are distinct from Joe's imperators who are not covered in white powder, though decorate themselves with black pigment, or grease. During their downtime, War Boys - wary of their short lifespan - carve, or brand mechanical autoparts onto themselves such as engines, pistons, etc...
They cover themselves in white dust to mirror the preferred stylings of their leader and carve intense—and one would imagine utterly painful—scars into their bodies.
It's painted white. You can see that Nux's skin gradually turns back to a normal flesh tone as the paint wears off through the course of the movie. There is still some left at the end, but most of it - particularly after he is splashed with water when Max first encounters Furiosa and the wives - is washed or rubbed off.