The girls' waitress outfits, if I look at them closely it kinda reminds me of Omega Quintet.
Not to mention Akari's work outfit is very similar to the main three girls' outfits in culture festival.
Each girl's waitress outfit has different color that represents cards symbols and it's exactly like in Omega Quintet.
It's attached on their left knees:
Red heart: Yuuki
Blue spade: Shana
Green club: Kotori
Yellow diamond: Akari
As in Omega Quintet's case (attached on their collars):
Red heart: Otoha
Blue spade: Kyouka
Green club: Kanadeko
Yellow diamond: Nene
There's also a 5th person which is Purple moon starred by Aria. Perhaps it symbolizes joker.