I've been a contractor myself and been around contractors for roughly 10 years. On base, it's a different beast and perfectly acceptable to dress formally as you would on the mainland. My experience is that 90 percent of base is not local and therefore they are used to the cultural norms of how you dress versus here. Some people visit and want to wear the aloha shirt because they can. In fact even while interviewing here, it's preferred that you wear a black suit compared to outside of base where aloha shirts are acceptable. They aren't shunned though and in fact you'd be less scrutinized wearing a suit (on base). I'd personally go with a jacket, no tie or no jacket, with a tie. No jeans. This is just my opinion based on my experience but hope it helps!