Looking through the Alara model does give a nice sense for an alternate way to do a five planar model, one which allows each plane to have a bit more nuance and overlap with other colors.I quite like the shards of Alara model and it has the benefit of already providing details of these planes of existence if you can find the original articles. Other planes are mentioned in the cards, some of which might make the basis for a plane of existence (for instance, Serra's Realm is a plane of floating earth motes populated by angels).
As for entities, using the base 1 colour model:
Blue: air/water elemental-typed (elementals, genies, etc). Also clockwork entities like the modron.
Black: any and all fiends, undead
Red: earth/fire elemental typed, chaos creatures (slaad).
There would certainly be a bit more nuance with more colour combos (for instance, maybe unicorns are celestials from the white/green plane and night hags are from the green/black plane), but for a basic 5 colour model this is how I would assign creatures to them.
This may be the easiest way to go about this. Start first with what denizens would exist in the basic five colors and then shift around as appropriate, if one chooses to expand it to other models.Yea, having a "type" of each extraplanar entity associated to each color would make a lot of sense. Like fey would be Green, and angels would be White.
I kinda like the MtG setup for planes as one can frame conflicts in terms of the color pie. For example, fey oppose fiends and modron, while fiends oppose fey and celestials, and celestials oppose fiends and dragons (possible chaos representatives), while dragons oppose celestials and modron, and modron oppose fey and dragons.