She captioned the snapshot, "More than a facial..."
In the image, we can see Nyong'o drenched in what looks like purple Nickelodeon slime, only she's not on the set of Double Dare. In the comments, people guessed that she might be doing prep for a movie, since the hue is reminiscent of something that would appear on Avatar. But a quick glance on the actor's IMDb page shows that her upcoming roles don't require any sort of full-body transformation. Star Wars: Episode VIII — The Last Jedi? Nah, she's full CGI in that franchise. Black Panther? Nope. No purple there.
"It's for a bust casting so they can make prosthetics," Instagram user @rabbid_liptick pointed out. If you've ever watched SyFy's competition show Face/Off, you may be familiar with facial prosthetics. They're used to alter a face in subtle ways (New nose!) and more extreme ones (Conjoined twin!). Usually, models can sit for a fitting, but if a movie requires the same ones over and over, it makes more sense to have a copy of Nyong'o's head on hand at all times.
By using a casting, Nyong'o won't have to sit for makeup artists every time that prosthetics are being used. Using a pre-made bust, the special effects makeup team can create anything they like and know that each piece will fit just right.
But as you can see, Nyong'o looks pretty great even when she's covered in purple gunk. Now all we need to know is how much she glowed after that purple stuff was washed off.