The Ender Pouch is a small bag that has a player-sized inventory added by the Ender Storage mod. Each combination of colors has access to a different inventory. Do note that in SMP, your items will be accessible to anyone with the same color coded Ender Pouch or Ender Chest
Depending on the server's settings, the Ender Pouch may have an inventory twice that size, or merely nine spaces large.
To set the color of the Ender Pouch, you will first need a correctly colored Ender Chest that is placed in the world. With the Ender Pouch in your hand, sneak + right-click on the Ender Chest to change the pouch color.
Alternatively, you could craft the pouch with the desired color wool to make all 3 of the buttons that color.