OK Magazine used the description given in Breaking Dawn to have a police artist sketch what the real Renesmee would look like.
"OK! asked a police artist to create a portrait of what the child of Edward Cullen and Bella Swan should look like, based on the descriptions of each character in the Twilight Saga. The artist’s rendering bears a striking resemblance to 10-year-old Mackenzie Foy, the actress who was cast as Renesmee.
“She sort of looks like Bella, which is absolutely right,” Kristen Stewart has said of Mackenzie. “Although the funny thing is she has green eyes and we both have to wear brown contacts together!”
The artist rendering shows Renesmee with green eyes, as Bella imagined her child, since Edward used to have green eyes. But as readers know, once Renesmee arrives, she’s inherited Bella’s chocolate brown eyes!"