Well, the main party is wearing all black because that is the signature color of the royal court of Lucis. It's tradition for them. I think it's actually a cool idea. The main party being all black and the world around them being all colorful and vibrant and strange. It makes them appear even closer knit together. Them against the world.
But after a while, and it has been a very long while, seeing them in the same black clothes gets old and boring. I too hope there will be some colorful stuff and/or silly stuff you can dress the guys in. I don't care if it makes sense for them to not wear anything beside black, I just want color and a little fun! >__<
I really like the extra costume system of the Tales of series and I would love something like that for FFXV:
But I doubt this will be possible. It's not just the colors, but also that stupid deal with Roen... >__<