The Canvas Bag is an item added by RedPower 2 that has an internal inventory. It has a total of 27 slots, capable of holding 1,728 items. To open the bag, press "Right Click" while holding it, and + "Left Click" or drag items in. You won't lose the items in the bag when you recolor it.
The Canvas Bag color can be changed much the same way you dye wool, by placing dye in the crafting table with the bag. Bags may also be recolored.