A kefta is a robe that only Grisha wear. A kefta's color is based on what class the Grisha is part of, and some are even made with Materialki corecloth which can withstand bullets. The embroidery on a kefta's cuff also signifies class.
Kefta Colors
Black - Only The Darkling wears this color, though Alina Starkov wears a black kefta at the party.
Blue - Etherealki
Red - Corporalki
Purple - Materialki
White and gold - Servants