Makeup influencer Tiara Willis, a teenager who focuses on makeup for people of colour, shared two screenshots of the ads — one taken from Stylenanda's website and the other from its Instagram Story — while pointing out the oddity.
"The graphic designer must've never seen a black hand in real life before," she wrote in a tweet that has since been shared more than 4000 times.
Other Twitter users shared her dismay, suggesting that the hand in the ad was either digitally edited or made to look darker by using makeup.
The DailyMail.com has reached out to Stylenanda and L'Oréal for comment.
"Did they really just dip a white hand in a darker foundation ... that's a yikes from me," one person wrote.
"They blackfaced a hand," someone else tweeted, adding: "Fix it Jesus" next to a string of face-palm emoji.
"It's crazy the absurd lengths these people will go to not actually use black people," another person tweeted. "They really just painted sis whole hand brown and think we wouldn't notice."
One Twitter user deemed the images "embarrassing", while someone else asked: "Why couldn't they just get a black model?? Is it really that hard?"
"Is this hand really doing black face rn? Idk how to process this," another person wrote.
Someone even pointed out that the hand's pinky finger seems to be a different colour from the rest of it.
"Jrbtxbzkcfkrkflssldgkrkd they just PHOTOSHOPPED THE COLOUR ONTO THE HAND I'M DEAD!!!!!!!!!!!! They forgot the pinky," that person wrote next to a close-up on one of the images, which shows the two intertwined little fingers.
L'Oréal, which is headquartered in France, acquired 100 per cent of Stylenanda in May, calling it "a highly desirable brand among Korean and Chinese millennials".
Stylenanda, which also sells clothes, shoes, and accessories in addition to makeup, was created in 2004 in Seoul by entrepreneur Kim So Hee, who was then in her early 20s.
By the time L'Oréal took over the brand, its makeup entity, 3CE, represented more than 70 per cent of its business.
The French cosmetics giant was planning to expand 3CE sales globally within two months of the acquisition in May, after securing the necessary approvals.