The Ender Pouch is a small bag that has a player-sized inventory. (Note: If you are playing on a public server, your items may be accessable to someone else.) This is like the Alchemy Bag in that it has multiple colors with a different inventory in each color.
Depending on the server's settings, the Ender Pouch may have an inventory twice that size, or merely nine spaces large.
For the most convenience, use both the Ender Pouch and Alchemy Bag, to know why read the category entitled "Ender Pouch v.s. Alchemy Bag", specifically, the subcategories titled "Uses" and "Bottom line", below.
To set the colour of the Ender Pouch you first need a correctly coloured Ender Chest and then have the pouch selected (in your in game hand) and right click on the Ender Chest.
Ender Pouch v.s. Alchemy Bag
The Alchemy Bag has 104 slots, while the Ender Pouch only has 27.
Their functions are the same, with the exception that the Alchemy Bag is player specific, while the Ender Pouch is server-specific and has more possible color codes than the Alchemy Bag. Alchemy Bags also have the ability to allow certain EE items to work when placed in them, unlike when these items are placed in an Ender Pouch.
Ender Pouch's Solution for Infinite Inventory:
The Ender Pouch coupled with an Ender Chest and Buildcraft Pipes can be used to make a player's inventory infinite. When you place items in an Ender Pouch, the items are also placed in the linked Ender Chest. By connecting a pipe system to the linked ender chest that will pump items from said ender chest to a different chest, will make the ender pouch never fill up, because every item the pipe system pumps out of the linked ender chest disapears from the ender pouch.
This infinite inventory solution is useful if you are mass mining blocks--ore, stone, dirt, sand, etc.--and don't care whether or not you keep them in your inventory.
Alchemy Bag's Solution for Infinite Inventory:
Equivalent Exchange (EE for short) achieves infinite inventory by placing an alchemy bag into a different colored alchemy bag.
This infinite inventory solution is useful when you want to keep items in your inventory.
Use both the Ender Pouch and the Alchemy Bag.
Use Alchemy Bag's solution for infinite inventory for storing items that you need in your inventory while away/at your base.
Use Ender Pouch's solution for infinite inventory for storing items that you don't need in your inventory while away/at your base.