While out of play (in deck or hand, for instance), you can target as either (like with a search ability). When you go to play it, you choose one side or the other, and that is what it will remain as until it is removed from play via bounce, shuffle, etc. If you play The Centurion at Calvary as a hero, he remains aligned to that side of the card. Even if he is converted to an Evil Character, he is just the converted version of the Red side of the card and does not "flip" to the gray side. That being said, if you play him as a hero, if he is bounced, you can replay him as an EC. Flip cards are fun like that.
