why wasn't galen's eyes yellow before his redemption?. i thought all dark jedi had yellow eyes. and while were on the subject why weren't dooku's eyes yellow?.
The Dark Side yellow eye isn't a permanent effect, it subsides. It has with Cade in Legacy, Anakin Skywalker, and Jacen Solo. It's just yellow-red while they are immersed in the DS.
I think it's a combination a a couple of things. First, like IllogicalRogue there said, it's kind of like when the Hulk gets angry -- yellow eyes tend to come when someone's completely let themself go over the the Dark Side. Second, I think Galen was more confused than angry most of the time. He'd do what Vader said, but he hadn't completely given in to the Dark Side yet. Unless you played the Dark Side ending, of course.
Not true: Bane. Sidious. Maul. Krayt. Wyyrlock. Talon. Stryfe... ALL of them have yellow eyes, ALL the time. Save for Krayt's mechno-eye, of course. And this list is not all-inclusive. Each of these examples fall in line with what I said earlier about Dooku and the fake apprentice, none were evil enough to maintain the yellow-eyes of the darkside. So I agree with you here, just not up above.
My personal thought for the "permayellow" eyes is that it happens to Sith who have been using the dark side for so long it changes their looks in a "Sidious is ugly as heck because the dark side has been rotting him" sort of way. Sort of a "I've been angry and evil for so long that I can't even hide it" type thing. I think Dooku was probably able to contain his anger pretty well and was dark for a pretty small fraction of his life. Unsuited Anakin wasn't truly dark for long enough. LoTF putting us inside of Jacen's head a lot shows us a man who flipped between relatively rational and calm to bat-pants insane. Galen...well....I think he was simply confused. He was portrayed a tad bit more powerful than I am comfortable with. When it comes down to it I don't think that Vader spent all that much time training him. With Worlds to subjugate and evil to do, who has time for an apprentice? He was sent around to do the Empire's bidding without really understanding what was going on, somewhat like Mara Jade as the Emperor's Hand.
But others didn't. Darth Sidious changed his eye color at will and Darth Vader's eyes turned normal just before his death on the 2nd Death Star. Plus Darth Tyranus' eyes didn't change to yellow. Exar Kun's and Ulic Qel-Droma's eyes didn't turn yellow as well.
Hm, I'd just probably chalk it up to him not always having evil intentions and/or using darkside techniques. Yeah, he's killing tons of people, but there's a good deal of conflict in him along the way. Possibly like Mara Jade, just more Force usage and more conflict, of course, while Mara was a good servant (of course, many of her missions were against corrupt officials or crime lords so she still had some intact morals). I think Ulic and Exar pre-dated the "darksiders have yellow eyes!" thing, didn't they? Oh well, maybe the KotOR comic will fix that (as we saw Exar Kun in a flashback recently). Sidious, well, he's walking evil, Vader is also pretty evil most of the time, or at least, he's a ruined man, while with Dooku he probably tries not to manifest the eyes in public (makes him look more sinister). Although I think he has yellow eyes in the cartoon series? Gotta dig out the Blu-rays later to check. Could simply come down to malice/evil intent and/or how much darkside power is being used (its hard to tell with Palpatine, whether his physical deterioration was due to the reflected lightning, or it had always been there and he had been hiding it with a physical or Force mask), and despite the Secret Apprentice's overpowered nature, he didn't really break out into megalomaniac laughter that much. Although I haven't played the game, but glanced through the novel and comic adaptation and didn't get that impression either. Although I still didn't really get the point of training him- Vader may or may not have wanted him to help him overthrow the Emperor, but the Emperor was in on the whole thing from the start? Meh, not that TFU's plot was ever all that good.
It's highly ambiguous with no established rules as yet. It seems likely the "dark side" use causation is a truism, and perhaps prolonged use or powerful use of the Darkside can lead to permanent "staining" of the eyes, However, even Anakin at the moment of his death didn't have yellow eyes...and Darth Vader after 20 years, should've been pretty darn stained. Probably best to say...yellow eyes = something going on with the darkside but not the opposite...something going on with the darkside =/= yellow eyes...it can be blocked by the user, or perhaps isn't a side effect all users must endure.
I still say Cades our best example- he was harnessing Dark Powers even the Sith wanted. And his eyes would change after use. When he'd stop it would dissipate. now how long the dissipation takes could change depending on prolonged DS use.
I call mystery of the Force. Or in OOU terms, "what suits a given story". That isn't meant as a cop-out, but rather that the Force is not necessarily governed by that strict laws of consistency. What sort of effects the environment of Dark Sites demonstrate similarly doesn't seem to follow any consistent rules, save a general "evil change". I believe RoT says darksiders sometimes exhibit such changes as yellow eyes. Which suggest some arbitrary mystical reason rather than fixed rules.