I dont get why Robert Knepper´s eye colour is changing.

Let´s say that´s a picture of Rob´s eyes below. His eyes aren´t just brow (mahogany) but he has an outher green ring followed by a mixture of that green with brown, which makes it appear gold. Now, depending on his mood and the light he stands in, those rings are waxing and waning. That´s why it sometimes appears that he has green eyes and sometimes just brown or sometimes even gold.