My hope for color is a variable neutral color appropriate for each unit, like the natural skintone of a unit (tan Marro, perhaps grey or white for a given Soulborg, etc).
I could tolerate a consistent neutral grey for all units if needed, but absolutely do not want Arena Of The Planeswalkers's solid faction colors (with the obvious exception of Jace's transparent phantoms, those turned out nice).
If it's anything like the Hasbro Pulse campaign, they will be color coded based on General.