Centaurs are usually White, IIRC, which makes sense in terms of the energy theming of Plains.Meanwhile, in Theros, it's very clear that the deities are designed around the 5 Colors and combinations thereof (5 big deities are pure elemental, the other 10 are dual-elemental deities, one for each of the 10 combos). And I guess there's 5 color cultures too:
Blue - Tritons
Red - Minotaurs
White - Leonin
Black - The Revived
Green - Satyrs
With the human city states and the Centaurs somewhere in-between.
The gods are the main dividers of color themes in the Theros cards, from what I've seen, with the city states providing a good bit of theming there as well.