A.  I see this being something with recessive genes.  It is as if there is a  conflict between recessive genes and there is no dominant gene present that is able to express itself.  The colors that typically show are blue, amber, very light brown and a gold.  It is possible to have dark brown, but rare (rarer than heterochromia itself as dark brown is dominant and would overrule).  The reason is tied to blue, amber and gold being deviations of recessive traits (most times).  
Heterochromia is a somewhat unique trait and I see it tied purely to genetic pairing.  I don't know that it is spiritual.  This phenomena feels more like a battle between the recessive genes, each fighting to show itself.  

Even though a short post I found it interesting. Thanks so much for the question!  It isn't something I've ever thought on or considered.

Blessings, 

Lynn