In the Twilight series, vampires' eye color is an indication of their dietary choices—specifically, whether they drink human blood or animal blood.
Red Eyes: Vampires who drink human blood have red eyes. When vampires consume human blood, their eyes turn a deep crimson color due to the presence of human hemoglobin in the blood they ingest. This makes their eyes appear red, reflecting their predatory and dangerous nature.
Gold Eyes: Vampires who choose to abstain from human blood and instead feed on animal blood have gold eyes. Animal blood lacks the high iron content found in human blood, which causes the vampires' eyes to take on a golden hue. This signifies that they have chosen a "vegetarian" lifestyle, respecting human life by not feeding on it.