Purple has pretty much been their unofficial color since debut, and these two colors are the colors of their two albums. The response has been really positive mostly, but if your just not feeling it, that's just preference I guess, nothing to really say about it.
The logo is sticking with the fantastical, fairy-tale theme that the fandom name has. Neverland, an obvious reference to the story of Peter Pan, is the name of the fandom because it's a place where people never grow old and Idle and their fans can be together and stay young forever. Which also works with their name as it's a play on words. Aideul, is literally children, but I-들 also represents a group of individuals coming together. So the castle is pretty fitting, I think.