Star Wars: The Visual Encyclopedia says that as "windows to the soul, eyes reflect the emotions harnessed by the Sith. When rage and anger seethe inside a dark side user, their eyes may burn yellow with a fiery-red rim." What that means is a Sith's emotions will be shown through their eyes.
Basically because he wasn't inherently evil, like Palpatine or Vader. Dooku was a former Jedi, and he left the Order simply because he had different views than they did. Not because he hated them and wanted to use the dark side. Generally, Dooku was a VERY civilized person, unlike some other Sith Lords.
The Jedi were known for their blue eyes, but when the Sith turned to the dark side, their eyes changed to a yellow hue. This is said to be one of the first signs that someone has become a Sith.
First off, notice Maul, Opress, and other Sith always have these eyes “on”. Some others don't have them at all, like Dooku, Snoke, Kylo, and Asajj Ventress.
Why didn't Anakin have yellow Sith eyes while fighting Obi-Wan in Episode III? It would stand to reason that as Obi-Wan emerged from Padme's ship, Anakin would go “off the charts” ballistic. He had already fallen hard to the Dark Side, killing younglings and his fellow Jedi.
89-90). Thus Othellos actions directly lead to his downfall. In Revenge of the Sith, Anakin is also responsible for his own downfall. His pride and anger cause him to kill his superior and betray everything he has sworn to protect.
Why didn't Snoke have yellow eyes? Because he wasn't a Sith, he was a strand-cast created by a Sith to corrupt young Jedi such as Ben Solo. Kylo Ren wasn't a Sith, he was a practitioner of the Dark Side, Snoke wasn't a Sith because he was a body used by Palpatine to be present elsewhere in the galaxy.
Because Ben Solo is never totally corrupted and remains a mix of both sides of the Force, he does not have the yellow eyes signature of those who have been fully consumed by the Dark Side. Similarly, "Darth" is a title bestowed only upon Sith Lords.
Born in 15 ABY on the planet Hyperkarn, Rey was the daughter of a young man and woman, whose names were lost to history. Her father, Dathan, was the cloned "son," a bioengineered strandcast, of the Sith Lord Darth Sidious, making her the granddaughter of the fallen Galactic Emperor and Dark Lord of the Sith.
Could a Sith live forever? Yes a Sith Lord can, and already has. Darth Andeddu created the Dark Rituals to transfer his essence from body to body to ensure eternal life. He was ultimately killed by his followers but he lived hundreds of years before that murder.
Dooku voluntarily left the light side behind and became Darth Sidious' dark side disciple, taking the secret name Darth Tyranus and leading the Separatist army. Dooku advanced Sidious's secret plot to take over the galaxy, but forgot that betrayal is the nature of the Sith.
Realizing that the Sith's greed led to their destruction, Bane created the Rule of Two. The Sith thenceforth consisted of only two Sith Lords at any given time; the master who embodied the power of the dark side, and the apprentice who craved that power.
Darth Jar Jar, known to most as Jar Jar Binks, was a Gungan Sith Lord. He survived the Battle of Naboo and became a Representative of Naboo, when he helped to give Darth Sidious emergency powers.
In the construction of a Sith lightsaber, individuals partook in a process known as bleeding. By ways of the Force, they would pour negative emotions such as rage, hate, fear and pain into kyber crystals. The process would result in the crystals gaining a distinctive crimson or red hue.
Dooku was actually responsible for the crash, but told Grievous that it was the Jedi who had sabotaged the shuttle. This in turn fueled his hatred of the Jedi. As a result of the enhancements, his organic lungs were irritated by some of the implants, resulting in his persistent cough.
Either way, while Vader can use most other Force abilities, Force Lightning needs to be channeled, and Vader's limbs are explicitly not connected to the Force. The Star Wars: Episode III - Revenge of the Sith Visual Dictionary confirms Darth Vader cannot use Force Lightning, and even suggests this is the reason why.
Ren's helmet was a symbolic reminder of his lineage as Darth Vader's grandson, serving no practical use in his life. Ren's helmet was designed to emulate that of his grandfather, the Sith Lord Darth Vader.
Solo was Force-sensitive and possessed prodigious strength by virtue of his Skywalker lineage, a trait that he shared with his mother, uncle, and grandfather.
According to The Rise of Skywalker: The Visual Dictionary (2019), Snoke's physical appearance was purposefully designed by Palpatine to ensure his species remained unidentifiable.
Snoke was a Force-sensitive humanoid genetic strandcast male who ruled the First Order as Supreme Leader during the New Republic Era.
The weapon had a kyber crystal at its heart, and emitted a yellow-colored plasma blade when ignited. The emitter matrix on Rey's weapon featured a rotating gear-like mechanism, which caused the emitter of the lightsaber to open up and allow the plasma blade through.
It's possible that Rey adopted the yellow-bladed lightsaber (or perhaps the kyber crystal conjured a yellow blade based on the changes within its wielder) in honor of those emotionally detached sentinels tasked with guarding the Jedi Order.
Luke Skywalker's lightsaber was changed from blue to green when they were filming because it was difficult to see. This became apparent during act one of Return of the Jedi when the battle on Jabba the Hutt's sail barge was shot during the day. The blue lightsaber blended in with the clear blue sky.