It's different kind of gold eyes. I dont think they're any way related to demons since different iris shapes and different shades too. Ntm that just bc they have similar colors doesnt mean they're related already. The key point to the demons is that their eyes are significantly different from that of humans especially their pupils
Also, with what they said about demons and religion, it's probbly safe to say that they all serve different gods and that the order/church of lumencis isnt even necessarily good too. So there's no concrete "good" or "evil" just different gods and in this case, lumencis seems to have gotten rid of the influence of other gods and changed the narrative to make it seem like those gods' followers are naturally evil entities while his (lumencis) is "pure" (questionably), while also reducing their existence into nothing and basically controlling knowledge surrounding the existence of other gods. If you think about it, it's kind of similar to modern-day christianity (with how they've demonized paganism and even hinduism).
so the two people who have ties with demons got gold eyes, the actual demon has gold eyes-
so tf is up with the blue haired, GOLD-EYEED girl's family? they all have gold eyes btw and also are extremely "religious"
probably overthinking it, but when the demon said it noticed many of its kind and similar to its kind in the world through her memories, it must mean that she had interacted with them at some point