Can Sith use lightsabers of any other colour than red or yellow?

Solution 1:

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)?

Solution 2:

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.