Each color crystal has a .uti file, but that's more for the line-item in you inventory than anything else.
The actual physical model depends on .2da entries and .mdl files, etc. It's been a gazillion years since I modded KotOR or TSL, so it's all a bit rusty, but I think I still have a tutorial around here somewhere that might be helpful.
EDIT: (slightly more helpful edition) Each lightsaber has it's own model with a color tied to it (i.e. lightsaber #1 is blue, #2 is green, #3 is red, etc). When you take the crystal (an inventory-only item) and put it in a lightsaber, the game takes what ever model (blue, green, red) you had away and gives you the corresponding model for the new color. Again, because all the hilts are the same, you don't really notice, however if you were to take or build a hilt mod, but only code it for one color, then that hilt would only show up in the game with the one color. Putting another color crystal in that "hilt" would then give you the generic one and/or probably crash your game.
Last edited by Achilles; 01-31-2012 at 06:05 PM.