We really don't know. We didn't even know what her name was until months after the final book was publish. I did read in an interview however, we can speculate by the name Greengrass, that Daphne has blond hair. Here's the link: Clicky-click

We can also assume because Scorpius has blond hair (a recessive trait) that Astoria more than likely has blond hair as well.

Although I have read some fics where Astoria has dark hair as well.

It seems completely up to you, though.