I think that is ivory(he kills tealgreen)because he said the same thing that says the note:goodnight aqua and tealgreen.
ivory kills him because they are enemies
I think that is emerald becase he buyed thing in the supermaket so he buyed the chainsaw and go to the roof and killed nils,but the person that followed rainbow and nils is amethyst maybe and after go to emerald and says the things that he saw.(the killer for me is emerald not amethyst)