I don't know if I'm just looking too much into it or does Kanekis eye get redder over time or even pitch black? In the manga, I've seen his eye all black, all black with red pupil, white with red pupil, white with big black pupil, as Haise I think I've seen him with a white eye and black pupil with a thick glowing red outline. Why does his pupil change colors if there's any reason for it?
They definitely change over time but this is most probably done either not intentionally or for the style. Unlike his hair, I doubt that it expresses any underlying meaning. For example, in darker scenes the color can be bright crimson or even scarlet but in some scenes (for example, Root A opening) it is light red and even close to pink.