Worlds KH: CoM

From Kingdom Hearts - Wiki on Neoseeker

All of the worlds of Kingdom Hearts: Chain of Memories are based on Sora's memories in the last game, meaning that they are the same ones as the Worlds in Kingdom Hearts. They all basically repeat the same general stories too. The difference of these worlds in this game, compared to their predecessors, is their structure. Each world is divided into rooms that are only accessible through doors. These doors can only be opened with certain cards that you pick after battling Heartless. There are three different colors of basic cards (red, green, and blue) and they all have numbers from 0 to 9 on them. To open a door, you will have to use the card of the right number, right color, or both. Some special cards will open rooms that are relevant to the advancement of the storyline or give you access to treasure. There are always three of these story-related rooms in Sora's version of the game, but Riku's Reverse/Rebirth will only have one per world. These special cards are always golden.

To break down the division of cards even further, it should be noted that each color has various types that will affect the condition of the room. Some cards will make rooms that have many Heartless to fight. Some reduce the number of Heartless. Some will cause them to be automatically stunned when you first enter battle. The conditions of the room, however, are not limited to how they affect the Heartless there.