he changed into ghost alpha.
Spoiler!!
Liu is a ghost alpha, when he knots with a beta, it releases chemicals that cause the beta's body to begin transforming into an omega. This process is called "changing". After this happens, a ghost Alpha's eyes change for a few hours; their pupils shrink, and their entire eyes turn a greyish colour, giving them a ghostly appearance. They are also more sensitive to light and have difficulty seeing clearly, which is why Liu is seen wearing dark glasses during daytime. Liu had never knotted with a beta nor any alpha or omega, he has seen to be mastering in controlling his pheromones and desires eventually leading him to be the "golden alpha", he never felt the need or desire to knot anyone. Until he met Yeehyeon.
But wasn’t it mentioned in many places that Yeehyeon was an omega but he didn’t know?
Yeehyeon is a Diamond Dust. Think of him like a hidden omega that only Weikun can smell the pheromones of. And Yeehyeon can only smell Weikun’s pheromones. He doesn’t sense any other alpha pheromones.
the first time they have penetrative sex, Liu knots Yeehyeon and initiates the Change but doesn’t realize he is doing it. It’s all instinct for him. He realizes what he has done, but continues to do it and doesnt tell Yeehyeon which leads to the main conflict/angst of the story. Liu is obsessed Yeehyeon (i mean who can blame him), and even though he is wracked with guilt for not telling Yeehyeon, he continues to Change him without his consent.
Does the color changing mean anything?