Shiro is there with Keith and is ready to get a little tipsy and dance with his husband. He's fittingly pastel by nature of having white hair, but also has a fondness for a white suit. And macarons. He'll eat a lot of those. He'll offer to help with any issues during the party too (he's a run and get ice if you need it kind of guy) but I'm assuming the princesses have this thing in the bag.
Noct came with his ~harem~. He's not very pastel but he did wear a purple suit and a flower crown. Despite being royalty himself, he's awkward as hell at these things so he'll just stick close to his people, dance when he's "dragged" and stuff his face with appetizers. Especially ones he can sneakily feed to one of the guys.