So, I've played through the prologue, chapter 1 and the first part of chapter 2. Now I'm on part 2 of chapter 2 on Ikalis' route (lucky for me he was the first choice on who's route to go down because he is the bestest boy) and I am really enjoying the VN so far!
I have two competing theories about Ehlly's purple eye colour. My first theory is that his eye colour means that there is a fourth crim, and possibly a fourth goddess, the knowledge of which might be not widely known on purpose or has been largely forgotten. The reason I think this is Ehlly's purple fur markings have magic in them, and also because of the existence of the heart and the book, which Ehlly's mom, Hilda, told him to hide.
My second theory is that his eye colour means that he has proficiency over both the body crim and the mind crim at the same time. My reasoning for this? A person who has proficiency over the mind crim, like Ikalis, has blue eyes, a person who's proficiency is over the body crim, such as Rodrick, has red eyes. If someone were to have proficiency in both, I think it's reasonable to expect them to have purple eyes as blue combined with red results in purple.
But that's just some theories. Some game theories! Thanks for watching!