Bastila primarily used a double-bladed yellow lightsaber. However, you can see the concept art of her using blue and green lightsabers. Either those were ideas that never came to life in the game, or she used those colors before starting to use a yellow saber. In the KOTOR game, you have the option of swapping out lightsaber crystals to change the colors of your lightsabers. You can do this for your party members as well. So, you get to choose which color she uses. When Bastila falls to the Dark Side, she unsurprisingly uses a red lightsaber.