I have a question. I am interested in all answers - canon, fan-theory or that I'm wrong or it should be hand-waved away from cannon. All opinions most welcome.

Why, when someone goes all Dark Side-y, do their eyes go yellow?

Good Anakin - blue eyes. Bad Anakin - orangey-yellow eyes.

Chancellor Palpatine - blue eyes. Emperor Palpatine - yellow eyes.

Good Ahsoka - blue eyes. Possessed Ahsoka - yellow eyes.

Also, Anakin's eyes go from blue (usual), to yellow (youngling killing time) and back to blue again (beaten by Obi-wan and having his legs cut off time). Palpatine has blue eyes when pretending to be good. Yellow eyes when Emperor and killing Mace Windus.

Asajj Ventress - always blue.

Darth Maul - always yellow.

What gives? What makes someone suddenly get a case of the Yellow Eye? Theories, explanations, please?

It's a visual medium and thus prone to being inconsistent?

Frankly, I don't think there really is a solid explanation. Though my guess is that it'd be an act of will on the part of the Sith as to whether their eyes go all dark-sider yellow or not. In the Revenge of the Sith novelization, there's dialogue about how Sidious will miss the mask that was Palpatine, as well as Vader later "putting on the face of Anakin Skywalker" as Padme's ship approached. Though in the film, Anakin's eyes were blue (far as I could tell) for his fight with Obi-Wan, but that's probably a case of the actor not willing (or able) to wear contacts during those sequences.

As for Asajj, she's not a true Sith (just has the trappings and some of the training) so she may not be prone to the yellow eyes. Maul has generally been tagged as one big ball of seething rage under tight control, so he's probably content to let his eyes remain yellow all the time since he's really more into the warrior side of being a Sith than the grand manipulator side that Palpatine embraced.

... Contacts...

But no, seriously. I'd have to re-watch the scenes you're talking about, but it might be just a simple case of more of letting go and giving into the dark side in full force. Sort of like in games and such where doing too much dark side will permanently warp the way you look, temporary eye color warping could be the first symptom until you start becoming fully engulfed by the dark side.

i think, as noted, it is visual choice much like the red skin and tattoos or red lightsaber. It gets the message across that they are the bad guys.

i think, as noted, it is visual choice much like the red skin and tattoos or red lightsaber. It gets the message across that they are the bad guys.

Okay, I get the meta-reasons. But what is the in-Universe justification for this?

i think, as noted, it is visual choice much like the red skin and tattoos or red lightsaber. It gets the message across that they are the bad guys.

Okay, I get the meta-reasons. But what is the in-Universe justification for this?

They're Sith. If you're looking for some biological reason then just use the corruption reasoning.

I always chalked it up to Dark Side taint.

Dark side corruption.

I think it may have to do with how far you've fallen. Palpatine was definitely evil doing some evil things like blowing up planets. Anakin...killed a crap load of kids. Asajj ...dark side but not entirely dark side...she didn't do anything that evil. Nor did Mual...assassin yes ...but whole planet and kids evil...not so much.

I'd definitely represent their fall to the dark side. The eyes getting more sunken and dark spots around their eyes. Pallid flesh, dark veins and eyes starting to redden. Subtle at first...you look more pale...eat something? Low blood sugar? Your eyes are blood shot...could be the planet or allergies...veins dark? Who knows...

Edit: let me add Dooku was dark side but not fully corrupted. He was massively manipulated and was disposable like Asajj and Maul. Manipulated and expendable. Not fully turned or corrupted.

But subtle hints until one dark day you announce to the PCs this guy...is EVIL...why are you with him?

It seems to be a side-effect of giving yourself over to the dark side. If you'll notice in Episode III, Anakin's eyes keep changing back and forth, being yellow on Mustafar after slaughtering the Separatist leadership and then blue again when Padme shows up. They go yellow when he nearly burns to death, but in the scene right before the helmet lowers onto Vader's face, they're blue.

I feel like it represents Anakin struggling with the dark side and his desire for power against an inherent goodness. "There is still good in him," after all. The fact that it flickers on and off is a sign of him wavering, but every time he could have renounced the path of darkness he chose to double down instead out of desperation.

Palpatine has yellow eyes at all times because that's what it takes to be a true Sith Lord. He probably had to expend effort to avoid his eyes showing yellow when he was Supreme Chancellor. (If he's a Sentinel Shadow like a lot of people here theorize, he's clearly a fan of the Shroud talent.) Darth Maul had them because he was indoctrinated from a very young age to draw from the dark side and only the dark side.

Dooku and Ventress don't have them, interestingly, but that speaks to their characters. Dooku really did believe in cause of overthrowing the Republic more than he believed in the dark side, and he saw his apprenticeship to Sidious as a means to an end. He was never able to draw on the dark side's full power, even though he was proficient with Force Lightning, and that led to Sidious conspiring to replace him with an apprentice more likely to do so.

Ventress, on the other hand, was too afraid to commit to the dark side fully, because there was still a piece of her master left in her mind. Even though she took the position that she did for revenge, it was a vengeance quest born out of love, and some part of her probably didn't want to disappoint her master's teachings. When she was betrayed in turn, it led to her giving up both sides of the Force and going underground. How well that sticks... well, we'll have to wait for the novel.

Finally, when Ahsoka's eyes turned yellow, it was because she was possessed by the dark side incarnate, as was Anakin later in the same arc. It wasn't a choice to give themselves over, rather it was forced upon them.

Also there are dark JEDI who are not Sith. They use the force to whatever means with wish. A dark Jedi may just be a hired gun or a bouncer or use his powers to gamble. He wouldn't be fully corrupted like Anakin-- who rolled over when Sheev told him to slaughter the F out of little kids.

Your PC may be harnessing the dark side because it's the force...plain and simple and dark or light it's his ally and if he uses it to save his allies who cares what side he's on. Dark Jedi. Bother term...grey JEDI.

So if your PC is slaughtering people without a whim batting kids off high balconies because they are begging for loose creds and creating giant space stations that destroy worlds...don't give him dark side corruption or not the really bad side effects. You NEED to gauge his actions. Kill kids? That's a paddlin', kicking puppies because? That's a paddlin'. Giant ships that destroy worlds? You better believe that's a paddlin'.

Dark Jedi just killing because they can at him and your crew and you used the dark side? Meh-- keeps using it? Maybe gets more pale but I wouldn't do the full blown dark veins, sunken face and eyes.