If you’re referring to Doyoon’s eyes then the change in colour has to do with his malnutrition at the start of the story. It was explained that his eyes turned red because he was hungry and hadn’t been able to satisfy his hunger for a long time because he couldn’t absorb energy from his partners, which is what landed him in the hospital eventually. His eyes returning to their natural colour came as a result of him drinking blood and being healed, which freed him from his malnourished state.
Thank you for telling me I barely remember them mentioning the reason but thank you very much.
Doyoon's eyes color are grey since chapter 6 (that's the first chapter where we get close up look of his eyes), so i think his eyes color is meant to be grey even since the beginning (it just seemed black from afar)
I like to look at the improvement from the first episode to the last episode and it was great but I'm wondering if anybody else saw the change in eye color?