The lightsaber colors come from the crystal inside the hilt. The color is chosen by the maker, but they are generally representative of the kind of Jedi the person is. Blue is the color of the Jedi Guardian, who are pretty much the warriors of the Jedi and specialize in lightsaber combat. Green signifies the Jedi Consular, who are the more force focused Jedi. Yellow represents the Jedi Sentinel, who are pretty much undercover, spy Jedi, who focus in other abilities. Ultimately you don't actually have to use the blade color representative of the branch of Jedi you are in. I believe Ahsoka was a Jedi Guardian, yet she used a green lightsaber.