I have an alien in one of my stories that bleeds. I want to use science to determine what the range of possible colors would be. It's an oxygen breathing alien, so it's blood has to be efficient at transporting it. I know our blood is red because we use iron to do that. I don't know enough about the chemistry of how oxygenation works to come up with any guesses for anything else. First off: What other atoms or molecules can bind so easily with oxygen? I know copper reacts with oxygen to turn green, how easy is that process compared to iron?