Well, since I did this update on warseer I see no reason to not do a quick one here.  I am going to copy and paste what I said on warseer:

The idea behind making squads with differing themes is to make the army look a little more dynamic and interesting on the table, and since I can couple it with the fluff I run for the army I see no reason to stop doing so. The idea behind this army is that they are "renegades." The platinum lord in the army made a special body he had developed prior to his transfer to metal so that he could break free of the C'tan once he was transfered. He managed to do this by making a deal with the deciever, a story I am in the process of writing in fact. Part of his ideal is to collect armies from other C'tan, or to find forgotten armies and bring them back into sanity as best he can. Each time he is successfull they try to bring back some of their culture by decorating themselves with thier world/clan colors. Hence the differing theme with each squad.