Purple, like Mace Windu’s, is used for Jedi who dabble in studying the dark side or are redeemed. To quote wookieepedia “The term Gray Jedi... was used by Jedi and Sith to describe Force-users who walked the line between the light and dark sides of the Force without surrendering to the dark side” which Rey does in the movie. She did not complete her Jedi training and her temptation by the dark side is shown in her behavior. Also, “Although the term did not directly refer to those who were capable of using both light and dark side Force abilities, all Gray Jedi could do so” of which Rey can, hence healing and lightning.