I think it's the iris that take the color of the blood. I was talking about the pupil. Humans have red eyes when you take a photo with a flash because you can see the blood on the retina through the pupil and I thought it would be the same with trolls. I did some research and found that animals have a "Tapetum lucidum" that make their eyes green, so it's probably what happened for Xefros ?

That was it! I remembered reading a post about it but I wasn’t completely sure about what name that thing had. And yeah it’s the Iris that takes on the color of their blood, but it felt appropriate to remark for the example.