It's a little bit of both. For instance, in this universe, if both of your parents do magic, then the aura ties into genes. You could get your parent's aura or your grandparent's aura and so on. There are other things that tie into aura color, however, and that is a change in personality or a sudden change in your moral compass. But we'll get to that in the second book.

When it comes to people who were reincarnated, however, they keep the aura they were born with when their soul was created. That is why Lunabelle still has a pink aura even after dying and reincarnating into someone different.