I have an idea. Humans, dwarfs, elves, vampires, and merfolk be the five dominate races in the world.

Elves will be Green/white
Humans will be white/blue
Merfolk will be blue/black
Vampires will be black/red
Dwarfs will be red/green

And that is if we want to go allied colors. If we want enemy colors.

Elves could be green blue
Humans could be white black
Merfolk could be blue red
Vampires could be black green
Dwarfs could be red white

The greater alliances could be made up of a mixing the two races that share a color.

Humans and elves should make Bant
Humans and merfolk be Esper
Merfolk and vampire be Grixis
Vampire and dwarf be Jund
Dwarf and elf be Naya

If we go in wedges with enemy duals:

Elves and merfolk make temur
Vampires and elves make sulti
Humans and Dwarfs make Mardu 
Merfolk and Dwarfs make Jeskei 
Vampires and Humans make Abzan