In the new editions of Magic Eventide what is the ruling on the dual color casting cost creatures and spells?

When I was playing this weekend I dropped a red/black creature with the dual casting cost. I tapped a RR and a colorless. I then dropped a fist of the demigod on it. On My interpretation of the rules the creature is considered a red/black hybrid creature but my opponent argued that since i cast it with a RR that it was a red creature only and not a red/black creature therefor a 2/2 when it was going to be a 3/3. Any rulings printed up would be appreciated