Spoiler warning! This article contains plot details from the latest books.
All elves, with the exception of Sophie, have eyes that are shades of blue, ranging from silvery blue to grey to almost purple. It is unknown why they all have blue eyes; it may or may not be explained further in the series. There are, however, elixirs that can temporarily change the color, and Dex can change his eye color to red sometimes; in Book 1: Keeper of the Lost Cities, he said that freaks everyone out. Sophie Foster is currently the only elf with non-blue eyes, and it's because of the fact that her DNA that is modeled after an alicorn's.