I wouldn't color the cards, I'd just leave them either sepia or black all the time. They are supposed to be sort of "separate" from the movie, after all. To color them would make the transition a little smoother, which I think is a bad thing in a silent film.
As for the omnidirectional on the cards...I don't know. I think it would be better to judge if we could see a scene in action with omni versus non-omni cards.