o.o well I read a couple of days ago in this KH wiki site something that might explain, although I dont remember this ever being explained in the game or by the director so its probably a theory. anyways, it says :
"Anyone who has traveled in darkness for too long would show a few physical traits: their skin darkens and their eyes turn amber. This is especially true for Master Xehanort, whose Heartless and Nobody both have tan skin and amber eyes. In the case of Ansem the Wise, it is not so severe: he only has amber eyes.
Maleficent is also believed to travel the Corridors, though she does not show the physical changes. This could be due to her trying to command the Darkness, instead of becoming one with it. It is also know that Pete travels the Corridors as well. He is known to be protected from the Darkness by Maleficent's magic.
The black coats worn by Riku, King Mickey, and Organization XIII are said to protect wearers from the effects of darkness, and from detection by those who use darkness. Thus, those who wear these coats can traverse Corridors of Darkness without suffering the negative effects."
I don't know...it doesn't really explain why does Saix have amber eyes but maybe, like Xigbar, they got them while they were still human and what not. Even though Xigbar's eyes could have always been like that. And the reason for the skin color to change sounds kinda...I dunno...stupid. But yea, I'm guessing its all related to darkness.(Usually is lol)
