What is a Grey Jedi? Essentially, Grey Jedi are those who walk the line between the light and dark sides of the Force, or who reject the control of both the Jedi Order and the Sith.
Aside from Qui-Gon Jinn and Ahsoka Tano, other characters have been dubbed Gray Jedi by fans, even though the term isn't canon. For instance, Kylo Ren — a.k.a. Ben Solo — is a Force-user who taps into both the Dark and Light sides of the Force. However, he's the furthest thing from balanced.
Anakin was a Gray Jedi; the problem was that no one around him was interested in understanding what that meant. He was light and had the capacity for great heroics; but through his great passion—his love and his rage—he also had the capacity for darkness.
Ahsoka Tano from Star Wars: The Clone Wars can also technically be called a Gray Jedi, due to her forsaking the ways of the Jedi, but still following a path of good.
There are two basic ways of describing Gray Jedi. The first is that they are Force users who walk between light and dark. The second is that they can be Jedi who don't believe religion is necessary to use the Force.
The Emperor's son sired a child of his own, Rey, his daughter who inherited Sidious' power in the Force, unlike himself.
Around 44 BBY, Jedi Master Qui-Gon Jinn was thought of as a Gray Jedi by some members of the Order for his frequent opposition to their demands. One group of renegade Jedi described themselves as "gray" even though they held to the same views as the Jedi Council on the subject of the dark side.
Was Luke a Gray Code Jedi? In Canon, by the Sequels, he wasn't a Jedi at all. In Legends, by Yodas Jedi Order standards, he would've been seen as Grey, yes.
Mace Windu is not a 'Gray Jedi. ' Not in canon nor in Legends did Mace ever veer away from the Jedi Order or Jedi Code. In fact, this over-reliance and dedication to the Code played a huge part in his and the Jedi Order's downfall.
Jedi Master Yoda is the most powerful of them all. This should come as no surprise, as he was the head of the Jedi Order in the prequels and was revered for his abilities during the original trilogy. Yoda's power is unmatched and was a real threat to Darth Vader and the Imperials.
One could theorize that he bore a child through some sort of space artificial insemination, or something closer to what we've seen in canon Star Wars material. In the canon comic Darth Vader No. 25, it is revealed that Palpatine somehow used the Force to manipulate midichlorions to impregnate Shmi Skywalker.
A crystal embodies the Jedi as the heart of the lightsaber, and pure kyber has no color until it's bonded to the Force user. This process likely resulted in Rey Skywalker's lightsaber being yellow.
Yoda, in no way, shape, or form, became a Grey Jedi. Simply because Grey Jedi, in the Disney canon, are no longer official. Even if he fit the standards of a Grey Jedi (which he didn't), he would have been called something else.
Arguably, some of the rarest types of Jedi were the so-called Iron Knights. These are members of the Shards species - crystals with a brain of their own. They were Force-sensitive and all trained under the tutelage of Jedi Master Aqinos. The Shards had droid bodies, similar to that of General Grievous.
The shading purple speaks to moral uncertainty, reconstruction, and recovery. Therefore, the individuals that used a purple lightsaber generally shared an affinity with both the Light and Dark sides of the Force.
Rey Skywalker, once known only as Rey, or as Rey Palpatine, was a human female Jedi Master who fought on the side of the Resistance in the war against the First Order. A former scavenger from the planet Jakku, her life was changed by the tumultuous events of the last days of the New Republic Era.
Billaba became Windu's Padawan and trained under the Jedi Master in the ways of the Force. During her training, Windu allowed her to study Form VII variant known as Vaapad. She eventually became a Jedi Knight, renowned for her skills with a lightsaber and favored Form III when teaching other Jedi.
In Legends, orange lightsabers were most often wielded by Force-sensitives who identified as Gray Jedi.
A grim Jedi Master with an amethyst-bladed lightsaber, Mace Windu was the champion of the Jedi Order, with little tolerance for the failings of the Senate, the arguments of politicians, or the opinions of rebellious Jedi.
Bottom line, Dooku never considered himself a Sith. And he wasn't; he was a man looking to change the galaxy in a way the Jedi were unwilling to try. And that is why he joined the Sith and became Darth Sidious' apprentice, Darth Tyrannus. When Dooku became Sidious's apprentice, he was already an elderly man.
Dark Jedi were Force-sensitives who were devoted to the dark side of the Force but were not Sith nor members of the Inquisitorius.
While the Jedi Order had five ranks, the Gray Order had seven, three of which were co-rankings which recognized a member's ability to teach and have apprentices. The Gray Jedi deemed that levels of mastery should be broken down into two separate ranks in an effort to ensure a recipient of rank was deserving.
Thus, while Qui-Gon was less powerful than Yoda, Palpatine feared him more.
Use by Jedi
Force lightning ability was not restricted to dark-siders. Jedi and other light-siders who were strong of will and character could learn this power without falling to the dark side—but its use was viewed as inherently corrupting, and most Jedi Councils forbade its use.
After years of training under Dooku, Jinn achieved the rank of Jedi Knight and—after successfully training a Padawan of his own, a farmer's son named Feemor—became a Jedi Master himself.