Purple Cartridge

This can be a tough crystal to find in the lower levels for you Mace Windu oriented Jedi and Sith. One known source is to check the weapons on the Specialty Vendor on your first homeworld (Coruscant or Dromund Kaas) for weapons which already have this crystal in them; buy the weapon, and then remove the crystal and place it in your weapon! Evidently Sniper Rifles seem to be the ones with purple crystals, at least in the last build.

Edit: In the last few builds, color cartridge and color crystal have been combined into the same item, which is why I post this comment here.