Red for the Kiil'n because it's a pretty high class, beloved colour there, and blurple for Ataeru because red, especially that really bright red, is actually completely illegal in Ataeru because it's used for very specific things (mainly for traitor marks and related symbolism). Sandals aren't really worn in the Kiil'n either, so that's another big difference.

As for his Ataeru outfit, despite the fact it's Allerick's casual an outfit like that would be seen as pretty expensive/fancy so anyone looking would know for sure that this guy is either a Big Deal or has a lot of money (atm Allerick is only one of those lol). The blurple especially is linked to dragons so a lot of people might even think arrogant when they see him. Allerick's got a loose understanding of colour culture in Ataeru, though, and mostly chose it for the aesthetic because he wasn't a fan of the more typical blues and greens. The paisley designs on both though are pretty typical in Kiil'n clothing, and that's probably another reason why Allerick chose that outfit because it reminded him of home.