In Star Wars the Old Republic, can a lightsaber-wielding Sith use a saber with a colour other than red or yellow?
The color of your lightsaber is dependent on your color crystal, and you should be able to get any color you desire, no matter if you're Sith or Jedi (or Dark or Light, the below is no longer correct).
Your Light/Dark side affiliation will affect your options though:
- Red (Forbidden to Light 1 and above)
- Blue (Forbidden to Dark 1 and above)
- Green (Forbidden to Dark 1 and above)
See this question for more information on lightsaber colors: What colors of lightsaber crystals are there (and where can I get them)?
The colour of your lightsaber is determined by the modifications done to the weapon at an upgrade station.
You can add or remove the colour crystal to your lightsaber, but you should keep in mind that different colour crystals provide different upgrades. Although the majority of the time you can find the same upgrade with a different colour setup.
Colour crystals are created by the Artifice crew skill.
Currently there are no limitations on what faction or force alignment can use color crystals. Some weapons are limited to a particular alignment, but you can easily put any crystal into a weapon you can use.
Previously, Purple and Black-Blue crystals were only available in Republic weapons whereas Cyan and Black-Red crystals were only available in Empire weapons. They could be transferred between characters in Legacy weapons however, so opposite-faction alts could obtain them. However, in Game Update 2.0 these crystals now drop from the Scum and Villainy operation (Darvannis) and can be reverse engineered by Artificers for a chance at a schematic, so it is no longer required to have an opposite faction alt to obtain one of these.
All of this is verified by personal experience, but for a good guide to all the end-game color crystals see this article on dulfy.net.
As for the list of all the colors possible, see What colors of lightsaber crystals are there (and where can I get them)? Blem's list is out of date / incorrect.
This answer was correct at the time of its posting, but changes made by BioWare have invalidated it. It remains for historical purposes.
It's not whether you're a Jedi or Sith, it's whether you've embraced a Light Side or Dark Side philosophy (IE, earned more than 1000 or so net Light Side or Dark Side points, enough to move you past 'Neutral')
"Neutral" characters (Less than 1000, more than -1000) can use any and all color crystals.
Light Side characters (+1000 and above) can not use red color crystals.
Dark Side characters (-1000 and below) can not use green or blue color crystals.
Yellow is neutral and can be used by anyone and everyone (but is harder to craft because its component crystals are uncommon gemstones rather than common color crystals)
The above applies not only to lightsabers but to all weapons that take color crystals.