I marked [spoilers] just in case. I suppose everyone watched TFA, but I hide TLJ.

So, in TFA movie Kylo Ren does something extremely dark: he kills his father (it looked like he did it). But he didn't get his eye color changed, like Anakin had when he had turned to the Dark side, the color of burning amber or red.

In TLJ

Two movies, two directors, did they drop the dark side corruption eyes concept? Or is it on purpose to show that he is still hanging there in the middle between light and dark?

Yeah even Hayden Christensen was more dark as Anakin in Episode II than Adam Driver as Kylo .

I was mad about TFA when Kylo took of mask (There were few laughs in cinema). Everything could be fine if his eyes were yellow at start, like Lana from SWTOR. That would be perfect.

An interesting point.

Since a few people here think that everyone must be trained on anything Force-related, maybe it was simply the first lesson that any Sith learned. Since there are no more Sith, no one's eyes will change ever again, unless a Sith Force ghost tells the secret, or the super secret first Sith temple is located.

From a lore perspective, I was always under the impression that dark side corruption is like anything else: different for different individuals. The yellow eyes may be an inevitable side-effect of DS usage/immersion, but not everyone exhibits them right away or constantly, or at all in their lifetime.

As far as the movies are concerned, it is probably an oversight or lazy costuming.

From a lore perspective, I was always under the impression that dark side corruption is like anything else: different for different individuals. The yellow eyes may be an inevitable side-effect of DS usage/immersion, but not everyone exhibits them right away or constantly, or at all in their lifetime.

 

As far as the movies are concerned, it is probably an oversight or lazy costuming.

Palpatine didn't get the yellow eyes till he electrocuted himself

Palpatine didn't get the yellow eyes till he electrocuted himself

I think he was able to hide his corruption, the same way he could hide his “Sithness” from Yoda and the Jedi Council.

As far as Kylo not showing it but Anakin did? I think it’s all because of the intent of the storytellers. Anakin was dropping fast but trying to fight it. Kylo is being held in a “will they or won’t they” romcom like relationship with Rey, and deformed, ugly eyed Kylo won’t fit.

I think he was able to hide his corruption, the same way he could hide his “Sithness” from Yoda and the Jedi Council.

 

As far as Kylo not showing it but Anakin did? I think it’s all because of the intent of the storytellers. Anakin was dropping fast but trying to fight it. Kylo is being held in a “will they or won’t they” romcom like relationship with Rey, and deformed, ugly eyed Kylo won’t fit.

That could be close. When the eyes change, that's when you know they're gone. Or something like that?

I think the key difference is intent and feelings, which I feel is partially the point of Kylo's character: For all his efforts to commit to the Dark Side, he still has too much heart and empathy, which are the biggest obstacles to mastering the Dark Side. The Dark Side is primarily fueled by negative feelings, not necessarily negative actions-- it's not enough to do harm, you have to truly want to do harm and to enjoy it. Someone who kills one person and relishes in the act will fall deeper into the Dark Side than someone who kills three people but is racked with guilt afterward.

As Snoke explained, when Kylo killed Han, it split him in two because he didn't fully "own" the act. And if you look closely at Kylo in TLJ,

And to draw a parallel to Vader, you notice how when he killed fellow Jedi and children--people he once counted as friends-- he didn't have the corruption, but when he was slaughtering the Separatist leaders--individuals he never had any love for-- that's when it happened? Because the Separatist leaders were the first victims Vader was able to murder without any inner conflict whatsoever.