Millie and Marcia Biggs are twins but one is black and the other white.
Amanda said people are 'quite amazed' at their daughters
They are the daughters of father Michael, who is black, and mother Amanda, who is white, and celebrated their first birthday on Tuesday.
The couple, from Erdington, Birmingham, said they were identical when born, which was within minutes of each other, but Millie gradually turned darker.
"They say it's just one of those miracle things. People are quite amazed," Amanda told BBC News.
The babies were conceived after IVF treatment, the Daily Telegraph reported.
Jane Denton, from the Multiple Birth Foundation, told the newspaper: "Two separate eggs were fertilised by two separate sperm.
"The genes that go into defining skin colour will be different which results in a combination of genes."
Mother Amanda added: "They were identical when they were first born and we first noticed a change when they were about seven weeks old.
"Millie started getting darker and Marcia was very light with her blue eyes.
"They are very close now and have really bonded well, they hug each other a lot.
"Millie's the mischievous one and Marcia is more chilled."