Remember in Juan's closing when he was gently describing Travis's death and what his last moments were like .... well ... when he referred to the "rainbow' as he was sliding against the wall, still standing, then went down to the floor. He spoke of the rainbow shape (with nothing good waiting for him at the end of his rainbow). Well I have always felt there was some kind of symbolism in there.
Anyway I did a little researching ... and maybe this has been talked about before, but ... the rainbow/iris (goddess) thing. It's very interesting to read because I think this murderer was very much into symbolism and hidden messages (cuz she like to be mysterious dontcha know). Anyway ... it's a quick read ... but very interesting IMO.
http://www.goddess-guide.com/iris.html
There are many other (more in depth) articles to google and read about the Goddess Iris ....
shape shifter was the thing that caught me the most.