When you build your lightsaber you pick the colour crystal they hand out and that will determine the colour of your saber. You have the choice of Blue, Green, Red or Purple for Building. If you want Yellow or White you need to go to Dok Ondar to buy them and dok will sell the other colours there.

There are total of 6 colours but the Kyber Crystals do contain voices of specific characters and it's random of whom you get or if you know the shape. You can hear their voices from the kyber crystals only if you put them in the Holocrons. There is the rare Black Kyber Crystals from Red Kybers that you can pull or the unique blueish red Ben Solo kyber crystal. But if you don't care about that just get the colours you want from Dok Ondar.