The Imperial Knights were considered Grey.
http://starwars.wikia.com/wiki/Imperial_Knight IMO there is no such thing as a grey Jedi..only a Grey Force User. Jedi serve the light side...in the extreme. Extreme uses of the Dark Side are what they worry about and know how to deal with. Army of red blade wielding Sith...they know how to deal with them. Subtle intrigue and subterfuge (Palpatine) they are lost. Sith are of course the opposite extreme. They have taken their passions to the point of it consuming their every thought. They are single minded in their focus on their objective... of course is what makes them so dangerous. Now Grey are of course in the middle but they can lean more toward one way or the other. For instance: Ahsoka is Grey, but she leans more towards the light. Assaj Ventress is grey (after her break from Dooku up to her death) but leans more towards the Dark Side.
I'm going argue to this with not everything the Sith do is in the interest of the Dark Side. A Sith may actually do something "nice" provided that it furthers their purposes. For instance, after Darth Bane wiped out the Brotherhood of Darkness, everyone believed that the Sith were extinct. He was discovered by some bounty hunters/mercenaries, but instead of killing all of them, he left one or two survivors. This purpose was to keep the
rumor of Sith alive with scattered reports. But being only rumor, it could also be quickly dismissed as such. This helped the Sith to remain hidden for 1,000 years.
A distinction should be drawn between Sith and Dark Side- There is a great passage in the Legacy Comics where Darth Talon says something like "what it means to be a Sith has changed over the millennia" Essentially the Sith is a culture, a tradition that is not necessarily evil or selfish, You certainly have adherents like Palpatine and Bane, but also Vectivus who differed in their view of what being Sith involved. And like any culture it changes e.g. Naga Sadows Sith empire is very different to Revan's Sith Empire or Vitiates let alone Palpatine or Krayt in terms of emphasis and key policies.
Exactly so.
So could there be a light Sith - given the variety of Sith over time and place I don't see why not arguably Darth Caedus attempted it.
According to the lines Sidious fed Vader, Order 66 would bring about peace. The Jedi upheld a corrupt system of government which should have been replaced centuries ago. The only way to "fix" it was to tear it down in one fell swoop. However, once in power, he showed his true colors.
A secondary issue is is there even a darkside at all or is the Force one indivisible whole and the 'taint' people feel in the force is just the emotional dye the individual force user has placed on a place, object or act. There is no real definitive answer in the canon or EU, and I think that ambiguity is a good thing.
And now we come to it. The Force simply
is. Just as a sword or gun is merely a specialized tool, it's inherent good or evil is only realized in the hands of the wielder. The same is so with the Force. Nobody raised an eyebrow when Anakin mercilessly cut down the Geonosians defending their droid factories. Who's to say they weren't just minimum wage security guards doing their job, and some psycho Jedi comes in and slashes the place up with his nosy girlfriend. But the Jedi are the "good guys", so they couldn't possibly be in the wrong. Right?
I find this categorization of Force use comparable to Freud's theory of the psyche. The Sith tend to follow their Id. Raw emotion usually results in great strength. Excitement, anger, fear. All can be characterized with a corresponding spike in adrenaline. But they can be unpredictable, and very hard to control. The Jedi are more in line with the Super Ego; follow the rules, remain calm, and do what is expected of you by society. These two mentalities are oil and water, and are never truly going to coexist. They Greys however, are suspension known as Ego. (I'm shooting the first person who references GotG.) They represent every shade between black and white based on what they bring into their personal blend. Qui Gon was a very light Grey. Ahsoka was a bit darker. Starkiller was darker still.