It is just me or that Madhouse seem to love the colour purple or blue? Baise ,Kikyo ,Kalluto all seem to have a purplish colour scheme.
Don't forget Killua. He's pretty much the embodiment of purple in this anime.
And so is the Zoldyck family by extension. Just look at their mini-arc after the Hunter exam. The atmosphere was pretty much painted with purple undertones.
I noticed they use a lot of purple (and blue) when showing us something dark or mysterious (e.g. Rhythm Echo in episode 7, Killua ripping Jones' heart out, Illumi's 'fight' with Killua during the Hunter exam, Zoldyck mini-arc afterwards (especially when Killua tells Milluki not to hurt Gon), Gon and Killua walking past Hisoka's nen when they reached the 200th floor, etc...)
I hope they don't use this in Yorkshin though. The atmosphere gets darker, but somehow purple doesn't cut it. I want it really
dark, like black night sky dark. Where Kurapika's red eyes will really brighten and contrast against the background.