We might be able to see it better if it was in the original aspect ratio instead of this absurd portrait format.
we frequently see crew members drinking (often coffee) in scenes in TOS. props were often a typical styrofoam cup painted a bluish-grey.
It's not the coffee, she smiled when she saw Spock's backside. She leaned back to check him out.
Kirk had a cup as well. Sulu was drinking tea on the Excelsior when it got hit by the Praxis wave. I've seen them with drinks in other scenes. The bridge isn't a restricted area for refreshments during duty.
Lifer's juice, as my grandfather called it after his Navy career
Everyone is focusing on the drink, but what’s also going on is that Nichelle Nichols is completely aware of where the camera is and then repositioning herself so her face is fully in frame.
Hydration is important! Other crew members should take note. (Surely the consoles are waterproof.)
