Edward’s eyes are a very prominent feature in the book; in fact, it is one of the first things that Bella ever notices about him. His eyes change colour very frequently, depending on how thirsty he is, or when he last fed. Bella initially notices that: "His eyes were black, coal black."(TW1) When Edward’s eyes are so dark, his thirst for blood is at the fore. When they are his lighter "golden" (TW12) or "topaz" (TW11) colour, that reflects that his thirst is sated and he has been hunting recently.
The shared colour of the Cullens eyes is not co-incidental. It is a hallmark of the "vegetarian" vampire who does not feast on human blood. We can see the clear contrast between their eyes and those of James, Laurent and Victoria which are "a deep burgundy colour that was disturbing and sinister." (TW18). This redness of the eyes indicates that these vampires feast on human blood. Meyer writes that, "If a Cullen were to drink human blood, his/her eyes would turn burgundy.
It takes about two weeks without blood for vampire eyes to go entirely black. If that Cullen then returned to an animal diet, his/her eyes would return to dark gold. Brand-newly created vampires are recognizable by their eyes, which are a vivid, bright red due to the massive amount of human blood (the blood that was already in the human at the time he/she was changed) that lingers in the tissues. That red fades slowly over the course of a year." (PC1)