A new look and feel to how you catch and play Pokemon.
Unlike previous games in the series, all of your battles in Colosseum are double. Meaning that you will always have two Pokemon out on the field when fighting. However, despite of this, your maximum party will still stay at six.
Another difference is that you cannot just wander around in grassy areas catching any Pokemon you please. No, in Pokemon Colosseum all of your battles will be against other computer generated players. There are NO wild Pokemon. In order to form a party, you will have to literally 'snag' your creatures from the frequent trainers you encounter. But once again not all Pokemon can be 'snagged'. Only a special series called 'Shadow Pokemon' can be obtained. Once you capture one, you can then purify it. It is recommended you do this as while Shadow Pokemon do have their own few perks, they cannot learn any new moves, level up, or be traded. Only after purification is this possible.
To purify a Shadow Pokemon all you really have to do is become friends with it. Battling often, traveling, and cologne massages are all things that will sooth your tainted creatures. Once it's shadow meter is down to zero, you can then take it to a special area in the game to be purified.
All in all, even though Pokemon Colosseum has both a story mode and a battle mode it is still a short game. It only took me one weekend to complete. This includes all the extras after the main story as well. I recommend if you don't want to spend too much money on it you rent it first. Either way though, it is a fun game if you just want something quick to pick up and play.