My assumption is that there’s some weird magic stuff at play there. None of the other parents’ eye colors get mentioned nearly as much, so I’m assuming there’s something weird going on with Julian, and the eye color is kind of an indicator of that. I mean it makes sense, considering how the MC is the same species as Julian (unless they’re human). If MC is anything other than Nephilim/Cambion, they wouldn’t be able to use control magic; the only reason they can is because they’re a keeper. So I’m assuming that Julian is using some crazy control magic enhancing stuff or something along those lines, since if MC was, say an elemental, it’d mean that Julian is one too, and elementals aren’t supposed to be able to use control magic. Plus it’s mentioned several times in game how good Julian is at control magic, even if he’s not a Nephilim/Cambion. Unless it was explained why non Cambion/Nephilim Julian could use control magic and I just missed it.
That’s just a theory though, it could be totally off base.
Also, I don’t think MC’s ethnicity would be decided for them. Like you said, it’s definitely possible for PoC to have blue eyes, even if it isn’t common. Julian could just be one of those people.