I know what u mean ivebren trying to take update shots of my tank and corals for 2 weeks even borrowed my dads new digital camera and it wouldn't do right
Using the "neutral color picker" in picasa might get the tank the right color if you mess with it enough. (I played with it some, it works once you find the right shade to click). It does mess up the rest of the photo a bit, so if you're going to include anything other than the tank in a shot, you'll need a more robust editor.
Apple released iPhoto for iPhone and it has been getting good reviews. I don't have it, but it might be worth looking into since your 4S is whatcha got.