They change color. When she is in the bar they are red. Next day in the shuttle they are yellow, on the enterprise they are white. I know, she could have 3 pairs of identical, but different color earrings, but... When she confronts Spock about not being assigned to the Enterprise they are yellow, but a little bit later when Kirk finds her on the ship they are white. In all that hurry she took the time to change her earrings? Where did she have the extras? It's not like her uniform has pockets. It seems like her earrings change color with her mood: red - happy, yellow - normal, white - professional.