Every human as well as monster or beast had an eye color. Some variants were quite common - black, brown, green, or even blue, but they were much more common than amber or gray ones.
However, the rarest eye color, unless you counted albinos with their red eyes, was violet or amethyst. This could be both a blessing and a curse.
In Evan's case, it was hard to tell which it was.
If another slave had been in his place, such a rare eye color would have been a real stigma, a mark on his back. With this, the people of the Einrich Family could easily figure out who exactly had attacked Astrid, because the eye color could not be changed.
However, for Evan, it was also a blessing in some ways. Even though this body had belonged to Kellan for most of his life, Evan had had time to feel the effects of the Enrich Family. Maybe not for long, but he was their slave, and Evan would never forget that until he destroyed the entire Einrich Family.