Just a clarification.
My friends have asked the same question as well as some of my reviewers.
Why are both Bella and Edwards's eyes red?
Ok to answer you question, it's because when Edward bit Bella he ingested her blood. Her human blood changing his eye color to the red that vampires get when they drink people blood instead of animal blood. Bella's eyes are red because of all of the blood that is left in her body from before the change.
Sorry for the confusion,