Responses
changdeok's eye color changing is just a stylistic choice. His actual eye color is meant to be blue, the author just doesn't feel like coloring it in all the time. You can see this same type of rendering in her other work sometimes.
It's kinda like how mangakas will add a screentone for characters with light brown hair, and sometimes for the same character they'll leave the hair blank/uncolored/white if they don't feel like adding a screentone.
What??! That's it??! It was tooo short!!! ╥﹏╥
Also I have a question changdeok usually have black eyes but why it turn light blue sometimes? I noticed that in S1 too