I think it follows the order they appear in OMEGA flowey, but reverse, so swap purple and green. Similarly the Dark Worlds also thematically follow Undertale locations but in reverse, examples being Card Castle > Home is castle (even the throne rooms have a very similar layout) and Cyber City > Core and MTT hotel (Queen's mansion doubles as a resort).

There's also a theory that excluding red each soul corresponds to one of the classmates with portraits, with green, blue and orange linked to Jockington, Berdly and Catti, so maybe this will be a hint that they might have chapters specifically focused on them, with CH2 being yellow and also focusing on Noelle.

Another interesting thing, the bead toy sprite apparently shows six beads, but the only colors that are visible are blue, green and orange, which might mean these three are the ones you can "remove".