As far as I always understood it Vader's yellow eyes were because of his fall to the dark side and had nothing to do with the sith themselves.
If that is so as I think then why does Kylo Ren who is also fallen to the dark side not have yellow eyes?
Science Fiction & Fantasy Stack Exchange is a question and answer site for science fiction and fantasy enthusiasts. It only takes a minute to sign up.Sign up to join this community
Not all who fall to the dark side that we see on screen have the yellow effect. E.g. Count Dooku.
Other dark side users without yellow eyes, include Asajj Ventress:
There is a notable difference between Kylo Ren and Vader, one was Sith and the other isn't.
From JJ Abrams:
Kylo Ren is not a Sith
Yellow/red eyes signifies a very, very deep corruption by the Dark Side of the Force. It can be masked, as shown by Palpatine, but I doubt that is the case with Kylo Ren.
Nightsisters are all Dark Side users, but use it in such a way that it does not corrupt them. Not even Mother Talzin who had her normal eye color. Tradeoff is that they can never be as powerful as a true Sith.
Count Dooku/Darth Tyrannus never fully delved into the Dark Side, only tapped into it on occasion. That is why he retained his own eye color. Reason for this is because he never truly wanted to become a true Sith. His "fall" to the Dark Side was due to political reasons.
Asajj Ventress may have been a borderline Sith apprentice to Dooku, but she already knew the teachings of the Nightsisters, had some training by the Jedi in terms of Light and Dark, and Dooku himself never strayed too far into the Darkness.
Savage Opress was imbued by the Nightsisters' magic. He may or may not have been corrupted. Hard to tell, as his natural eye color already was yellow/orange.
Snoke also does not have yellow eyes. He could be masking it, or he's using the Dark Side in another way unseen before, similar to the Nightsisters but possibly more powerful. It's also been said he was looking for the perfect balance between Light and Darkness.
That leads us to Kylo Ren. Snoke said Kylo's training was not complete, and he's also expressed his opinions about Kylo Ren being a perfect apprentice due to him being a mix of Light and Dark. So Kylo's training may be more of a Grey focus. But at this point, it's too early to say. Though one thing is for certain: Kylo Ren/Ben Solo does not have yellow eyes because he's not "corrupted" by the Dark Side.