Frankie said on NeoGaf that there will be a reason why some Prometheans are blue and others aren't. Seems more complex than just different classes. If it helps, one of the Bulletins on Waypoint described the purpose of the Crawlers as being security guards to defend and control "Requiem's emerging threats." Suggesting there's something there that they want to stay there. Then I thought about Primordial's final line: "It is your destiny to destroy this servant, that another may be freed." I don't think we should ignore the possibility that Requiem may house this other servant.