I remember someone posted themselves doing the Erdnase color change very quickly during a routine and people were discussing various subtleties for the move, but I can't find it...

http://www.youtube.com/watch?v=hucYEoGgZUA
My 2 cents: by pushing the card forward a minimal amount (I go just about a centimeter) and using a deep grip, the movement of the card can be made almost invisible. My pinky and ring fingers are forced a little further back than what feels comfortable and my index and middle fingers have more room to move with the card naturally. With this grip, my index and middle fingers can follow the corner of the card as it gets pushed forward and back, eliminating the usually blatant discrepancy in the distance between the index and middle fingers during the change.