My guess is the dark one is a roo - standing more upright, thickness of the legs. I would guess the lighter chick to be a pullet - seems to be a bit more feathered out, doesn't seem to have the "cockiness" of the darker one.
i think the black one is a roo as he or she is very dominante and can be aggresive at times.we are now getting worried about the lighter coloured chick as it is getting a redish colour around its beak:/