I'm not sure, but I think it's a golden alpha thing. Like, after knotting someone, he can't see anything like he's literally blind (I don't know why though) but that's why he wears sunglasses in the car, which is useless.
As far as I understood from the other comments... (spoiler ahead, please don't quote as it ridicules the spoiler warning)
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His eye colour changed because he is not only a Golden Alpha, he is a Phantom Ghost who can only smell one perticular person and that may be or must be a beta and through knotting he changes that beta into an omega. The beta is – as far as I understood – some rare version, called Diamond Dust (you see what the author did here), which can only be changed by a Phantom Ghost into an omega. This change from beta to omega takes some time and it's beginning is indicated by the change of the eye colour. Maybe the process of the change is fastened by knotting more or even have more sex, I can't say that yet.
This is just what I understood out of the comments novel readers would leave on other topics. Maybe I got it wrong, I'm sorry then ⁄(⁄ ⁄·⁄ω⁄·⁄ ⁄)⁄
Wow
Oh damn that is wild! Thanks for the explanation (๑•ㅂ•)و✧
Spoiler
So from the novel I read, as a “ghost,” he can change any beta into an omega. This is why he worked so hard to become a golden alpha, so he would never make the mistake. The “Diamond Dust” is a specific beta that emits pheromones for the ghost. So, pretty much their fated pair. Diamond Dust isn’t the official term. It was a name that one of Liu’s ancestors made to their beta. It was a comparison term he used to the “diamond dust phenomenon.” The change takes a long time to occur. Repeated knotting over a long period of time. His eye color change happens after he knots and the reason why they’re called “ghosts.” His eyes become sensitive and hazy, so he wears the sunglasses afterward until his eyes turn back to normal.
Thank you for the additional information, Jabba the Cutie (▰˘◡˘▰)
Does anyone know why his eye color changed?