Well, I grew up believing that you can attune your lightsabercrystal to yourself, but that doesn't change the colour, which gets determined by the type of crystal you use, ie Ilum crystals are blue or green, artificial sithcrystals are red, etc.
And the Jedi tend to give blue crystals to guardians, green crystals to counselors, etc., but theoretically everyone can use whatever colour he or she likes (except black, because black is the absence of light and lightsabers without light are just shiny hilts).
And that's what I'd prefer Disney Star Wars to be.
I haven't read the new books, but in Rebels, doesn't Ezra get a crystal which is blue to begin with? Even before he can influence it with the force?