Those Chitons are purple because they eat a lot of coralline algae. I have some and they are prolific (reproducing in your aquarium). They will strip a rock bare, which I don't mind because I have coralline coming out my ears.
LOL Thanks guys! I have thousands of those little brittle stars. I didn't know much about that chiton deal so thank you for bringing it to my attention. I have plenty of coraline for them to munch on!