At the end of the 2011 Hunter × Hunter (episodes 27 through 58), there is a little short "Huncyclopedia", which is Gon and Killua talking about some character or element in the series.
From episodes 37 through 44, the two have a "Whale Island" theme, which shows them in shorts and out in the wild rather than in a classroom. However, in the two episodes following this, their skin tone is significantly darker. Oddly enough, they are still in their original tones for the intro, as well as when they make animal faces at the end of episode 46.
Is this supposed to be a tan, or is there some other reason for this change?