What makes sense to me is the taking into consideration the degree of establishment in the universe and the prevalence of some factions schemes and options regarding subfactions than others.
For example, Space Wolves and Blood Angels are decades old schemes which have had separate books and models for many years.
In Soulblight Gravelords, I can take Belladama Volga, Matriarch of the Vrykos in my Legion of Night Army. Neither faction has an established colour scheme, and skeletons are skeletons (though in the legion of Blood section it does detail that Neferata loves to paint her skeletons with tatoos and dress them in fine reds and the like so I'm waiting on a TO to tell me my skeletons aren't pretty enough to be considered LOB xD).
I think once the universe is far more established then colour schemes can play a bigger and more official role in WYSIWYG, until then the comparison with 40k and AoS (the former having much more establishment across time, books and models) makes a case that while I can see the argument for colour schemes in 40k, in AoS GW needs to 'earn' their proxy colour scheme rules by developing and establishing the game for a longer period of time.