Multiple realities
(covers information from several alternate timelines)
Nail polish was a chemical substance that women coated their nails with to look fashionable. (TNG: "The Quality of Life")
In the alternate reality, Carol Marcus wore polish on her fingernails that matched the blue color of her sciences division uniform. (Star Trek Into Darkness)
In 2365, a hologram of Madeline in the Dixon Hill holoprogram wore bright red lipstick and nail polish. (TNG: "Manhunt")
In deleted scene 36B of the episode "The Chase", a female buck-toothed alien was wearing nail polish while contributing a dermal cell sample to Dr. Crusher in sickbay.