Kalani and Jarani Dean, twin girls from Quincy, Illinois, turned 1 at the weekend.
Over the past few months the pair have built up quite a following on social media after their mother started sharing pictures of the girls - one of whom, Kalani, has light skin and blue eyes, while Jarani has dark skin and brown eyes, the Daily Telegraph reports.
The pair are fraternal twins; mother Whitney Meyer is white, and father Tomas Dean black, hence their unusually distinct appearance.
Speaking to People earlier this year, Meyer described her daughters as "miracle babies" who show the world "why racism shouldn't exist".
"They are different colours, but sisters by blood," she said. "But we love them the same."
Both parents were "totally surprised" to see the difference in their daughters' skin colour when they were born on April 23 last year.
"When Kalani came out I thought she (had albinism) because she was all white," said Meyer. "I asked my doctors, but they said, 'Nope!' And I kept thinking she would get colour but she didn't. It's unusual."
Speaking to the BBC in 2011, Geneticist Dr Jim Wilson of the University of Edinburgh said the chance of a mixed-race British couple having twins with different skin tones was in the region of 500-to-1.